A case is described of an autoimmune oophoritis that was diagnosed unexpectedly after a hysterectomy and bilateral salpingo-oophorectomy had been performed on the suspicion of ovarian cysts. The patient was a 43-year-old multiparous woman who presented with vaginal bleeding and lower abdominal pain which she had had for one month. Grossly, the ovaries were enlarged and multicystic. The cysts measured up to 3.0 cm. The major histological change was a lymphoplasmacytic infiltrate in close relation to the theca interna of developing, cystic and atretic follicles, but sparing the primordial follicles. The infiltrate increased in density with the follicular maturation and culminated against the corpus luteum. With involution of the developing follicles, the inflammatory infiltrate subsided to some extent. The proportion of the plasma cells increased with the density of the infiltrates. Immunohistochemical study of the ovarian mononuclear cell infiltrate revealed a mixture of B- and T-lymphocytes. The plasma cells were polyclonal. These histological features of the present case are typical of autoimmune oophoritis although the presence of autoantibodies and hormonal level in the patient's serum were unknown. This case may be identified as in the early active stage of autoimmune oophoritis.