A total of 128 patients with adenomyosis were examined clinically and histologically to investigate the pathogenesis of subserosal adenomyosis. In this study 15 (11.7%) cases were classified as subserosal adenomyosis on the basis of localization of heterotopic endometrial tissues that consisted of glands and stroma. These were subclinical and incidental. The uterus was not enlarged in this type of adenomyosis. Distribution of the ectopic tissues and the patients' mean age at the time of the hysterectomy suggest that subserosal adenomyosis may develop as a variant of pelvic endometriosis.