The aim of this study was to determine the expression of endothelial nitric oxide synthase (eNOS) in leiomyomatous tissue as a possible mediator of the growth process. Nine patients had myoma enucleation during the follicular phase. The myomata and adjacent myometrial tissues were fixed in formol until study. Immunohistochemical localization of leiomyomatous and myometrial tissue eNOS expression was performed using specific monoclonal antibodies to eNOS. Statistical comparisons were made using the Mann-Whitney Wilcoxon rank-sum test for the expression of eNOS. The test was considered significant when p values were <0.05. Immunostaining for eNOS was seen in the cytoplasm of myometrial endothelial and smooth muscle cells in both tissue sections. eNOS expression was significantly higher in the smooth muscle cells of leiomyomata, compared to parental myometrium (p < 0.0005). The expression of eNOS in vascular endothelial cells was not different in the leiomyoma and myometrium (p > 0.05). To our knowledge, this is the first study to document marked expression of eNOS in leiomyomatous tissue, compared to parental myometrium. We also conclude that the mechanism(s) of the growth-promoting effect of estrogen on leiomyomata is mediated by more synthesis of NO. Copyright (C) 2000 S. Karger AG, Basel.