The activities of six enzymes involved in energy metabolism were measured in leiomyoma specimens and in the adjacent normal myometrium from the uterus of 17 patients. In leiomyomas the specific activities of hexokinase, lactate dehydrogenase and hydroxyacyl-CoA dehydrogenase (HAD) were higher than in myometrium. The soluble protein content was lower in leiomyomas. Therefore, most of the differences of specific activities were not found when the enzyme activity was expressed per gram wet weight of tissue, except for HAD activity, which was still higher in leiomyomas (p < 0.05). This result is compatible with increased fatty acid utilization by leiomyomas, and with the increased growth potential of such tumors.