We describe 10 uterine leiomyomas with hydropic changes that created problems in differential diagnosis. The patients ranged in age from 41 to 51 years (mean, 45 years). Their presenting symptoms did not differ from those of patients with typical uterine leiomyomas. The follow-up, available for eight patients, was uneventful. Gross and microscopic examination revealed otherwise typical or cellular leiomyomas with focal accumulations of edema fluid (hydropic degeneration), typically associated with variable amounts of collagen ("hyaline degeneration"). Significant accumulations of acid mucins (myxoid degeneration) were present in only one case. These changes resulted in one or more of the following: (a) a characteristic perinodular hydropic change that mimicked intravenous leiomyomatosis on both gross and microscopic examination; (b) hydropic changes extending beyond the confines of the leiomyomas that raised the suspicion of myxoid leiomyosarcoma; and (c) extensive or subtotal obliteration of the usual architecture of the leiomyoma, often accompanied by numerous thick-walled blood vessels, that tended to obscure its smooth-muscle nature. Awareness of these changes combined with mucin stains and immunohistochemical stains for endothelial antigens should facilitate the diagnosis. Hydropic leiomyomas are clinically similar to typical leiomyomas; it is obviously important that they not be confused with intravenous leiomyomatosis or myxoid leiomyosarcoma.