In both rats and humans, partial hepatectomy results in a substantial increase in serum estradiol concentrations. Because mammalian liver of both sexes contains estrogen receptor, it has been suggested that this surge in serum estrogen concentration may initiate or facilitate liver regeneration. We have reexamined the potential role of sex steroids in human liver regeneration by measuring serum estradiol and testosterone concentrations in men and postmenopausal women before and after hepatic resection. The results were compared with those obtained in patients subjected to other forms of major surgery. In both men and postmenopausal women, serum estradiol levels increased after partial hepatectomy. However, the magnitude of increase was not related to the amount of liver removed. Moreover, similar increases occurred in patients undergoing other major surgical procedures not involving the liver. Serum testosterone concentrations decreased in men and increased in postmenopausal women undergoing all forms of surgery. It is concluded that effects of partial hepatectomy on serum sex steroid levels are nonspecific and probably related to stress. These observations diminish the likelihood that such changes are an important factor in the control of liver regeneration in humans.