PURPOSE: To evaluate changes in the MRI findings of pituitary adenoma after gamma-knife radiosurgery, and tocorrelate these with the clinical outcome. MATERIALS AND METHODS: We evaluated the MRI findings of 27 patientswith pituitary adenoma before and after gamma-knife radiosurgery. Their ages ranged from 16 to 69 years(M:F =8:19), and there were seven cases of microadenoma and 20 of macroadenoma. In 11 cases, the tumor was partiallyremoved before radiosurgery. The follow-up period ranged from three to 43 months. RESULTS: In two cases ofmicroadenoma, complete regression was seen after three and 12 months, respectively, with normalization of hormonelevels and the improvement of symptoms. Among the remaining cases, in which no volume change was noted, hormonelevels normalized in four. Among 20 cases of macroadenoma, volume reduction was seen in 18 patients and completeregression, in two. In cases in which the volume was reduced by 20%, the mean follow-up period was 6.6 months, andin cases in which this was 50%, the period was 15.3 months. On postenhanced T1W images, the most frequent patternswere rim enhancement (n=10;50%) and homogneous enhancement changed to rim enhancement (n=4;20%). In six of ninecases(66.7%) of hyperfunctioning macroadenomas, hormone levels normalized, and in nine of 11 cases (81.8%) ofnonfunctioning macroadenomas, symptom improvement was noted. In six of 20 cases in which the enhancement patternchanged after gamma knife radiosurgery, there was clinical improvement. Among 14 cases in which there was nochange in the enhancement pattern, nine showed clinical improvement but five did not. The former showed a meanvolume reduction of 62.5% and the latter, 16.5%. CONCLUSIONS: After gamma-knife radiosurgery, clinicalimprovement in microadenomas can be expected, regardless of volume change. The volume of macroadenomas decreasedand a rim enhancement pattern was observed after gamma-knife radiosurgery. A marked reduction in the volume of atumor, or a change in its enhancement pattern may indicate clinical improvement.