OBJECTIVE: To examine the ultrastructural characteristics of extracellular matrix and mature collagen fibrils in uterine leiomyomas and compare them with those in adjacent normal myometrium. DESIGN: Analysis of paired leiomyoma-myometrium in surgical specimens. SETTING: Research center and tertiary care center. SUBJECT(S): Women undergoing medically indicated hysterectomy for symptomatic uterine leiomyomas. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Appearance and spatial orientation of the collagen fibrils in leiomyomas compared with myometrium. RESULT(S): Observation of specimens at x 12,500 magnification indicated that collagen fibrils were more abundant, loosely packed, and arrayed in a nonparallel manner in leiomyomas compared with myometrium. Random areas were examined at x 6,500 to x 64,000 magnification and revealed collagen fibrils of equal diameter in both leiomyomas and myometrium. However, an ordered and regular barbed appearance was present in collagen fibrils from myometrium but was lacking in leiomyomas. CONCLUSION(S): Leiomyomas contain an abnormal collagen fibril structure and orientation, which suggests that the well-regulated fibril formation in myometrium is altered in leiomyomas. Alterations in collagen genes may play a role in the pathogenesis of leiomyomas.