To evaluate the accuracy of magnetic resonance imaging (MRI) for diagnosing nodular adenomyosis by percutaneous myometrial biopsies. Prospective observational study. Gynecology department of community hospitals. Twenty women with severe dysmenorrhea, chronic menorrhagia, and an MRI diagnosis of adenomyomata. Several laparoscopically guided, percutaneous myometrial biopsy specimens were taken in an attempt to confirm histologically an MRI diagnosis of adenomyoma; and resectoscopic endomyometrial biopsy specimens were taken in an attempt to confirm an MRI diagnosis of adenomyosis. Eighteen (90%) of the 20 women had an MRI diagnosis of adenomyosis histologically confirmed by myometrial biopsy. The remaining two (20.0%) had an MRI diagnosis of adenomyosis histologically confirmed by endomyometrial biopsy. An MRl diagnosis of adenomyoma was confirmed by transabdominal uterine biopsy in most patients. These results, when combined with those obtained by resectoscopic endomyometrial biopsy, established a diagnosis of adenomyosis in all patients.