Studies on menstruation have focused only on menstruation itself and menstrual disorders. The menstruating girls or women have been neglected. So, the purpose of this study was to understand menstrual experience of adolescent girls in their perspective and build a theory on it. The specific purpose of this study were to find initial reaction of the girls, their strategies to adapt to menstruation, consequences of their efforts, influencing factor, and patterns of experience. The subjects of this study were eleven adolescent girls who experienced menarche three months to twenty-six months before the interview time. They were selected purposively. Their ages were in range of twelve and sixteen. One of them was a elementary school girls, three high school girls, and seven middle school girls. Two girls were handicapped because of cerebral palsy. All of them had some knowledge about menstrual physiology and hygiene during menstruation. Data were collected from September, 1994 to July, 1995. Data collection and analysis were done according to the grounded theory methodology by Strauss and Corbin(1990). Data collecting method was the long interviews and observation. Each interview took from 1 hour to 2 hours. Interview were tape?recorded and transcribed later by author. Data were analyzed immediately after interviews. Based on the results of previous interview, next interview were planned until gathered data reached the saturation point. Results were as follows. One hundred and six concepts were found. Those concepts were grouped into twenty eight categories and then fourteen higher categories. Twenty eight categories were as follows, "want to hide", "bewildered", "sense of burden", "sense of heterogeneity", "gladness", "sense of superiority", "negative empathy", "positive empathy", "limited hygenic control", "sense of timing", "lack of knowledge", "lack of support", "advance knowledge", "informational support", "emotional support", "edurance", "prayer", "disclosing", "avoidance", "diversion", "sense of powerlessness", "discovery of sex identity", "sense of maturation", "sense of stability", "acceptance of menstruation". Fourteen higher categories were as follows, "negative feeling", "posive feeling", "exchange of feeling", "limited hygenic control", "sense of timing", "accumulated experience", "dysmen-orrhea", "level of knowledge", "need for support", "perceived support", "sharing of feeling", "self-control", "passive acceptance", "active acceptance". The core category was "emotional shaking", which consisted of "positive feeling" and "negative feeling". "Emotional shaking" comes up to every adolescent girls experiencing menarche, independently of any contextual conditions, and its demension has two directions: positive one and negative one. Its influencing factors were time of menarche, advance knowledge, support from the significant persons, expression and self?regulation. Even if they showed different process of adaptation to menstruation, general process of adaptation were as follows: 1. stage of emotional shaking 2. stage of acceptance 3. stage of internalization of the menstrual experience. Seven patterns existed on the process of adaptation to menstruation after menarche. Those are as follows. 1. If girls thought their menarche came too early and they had not much knowledge on menstruation, they had a kind of negative feeling. If they did not get enough support and dysmenorrhea superimposed, they came to accept menstruation passively. 2. If girls had menarche too early, they had negative feeling, even though they had enough advance knowledge. But support helped them accept menstruation easily. 3. If girls had menarche too early, they had negative feeling, even though they had enough advance knowledge on menstruation. But by experiencing subsequent menstruations and disclosing feeling, they began to accept menstruation. 4. If girls had menarche too lately and they had enough advance knowledge on menstruation, they had positive feeling. If dysmenorrhea superimposed later, their feeling turned in to negative one. But they came to accept menstruation positively by disclosing feeling and getting support. 5. If girls had menarche too early, they had negative feeling, even though they had enough advance knowledge on menstruation. In addition to this, if dysmenorrhes superimposed while they did not get enough support, they felt powerless and came to accept menstruation passively. 6. If girls had menarche too early and did not get enough advance knowledge, they had negative feeling. But disclosing feeling and support made them get sense of homogeneity and began to accept menstruation. 7. If girls had handicap, they had negative feeling, even though they had enough advance knowledge and menarche was late. But Menarche made them get feel sexual identity. Their limited hygenic control and negative empathy from their mothers made them accept menstruation passively. To let adolescent girls take their menstrual experience as a part of their lives forming a positive sense of feminine identity, it needs qualified teaching and, support and deep concern of the significant others. Nurses including school nurses should try to develop an educational program, which include menstrual physiology, hygiene during menstrual period, meaning of menstruation and impact of menstruation on the development of female sexual identity.