30 patients undergoing regular, three times weekly hemodialysis were treated with large doses of intramuscular testosterone with evaluation of hematopoiesis before and after treatment. A control group of 30 patients not using the drug was evaluated in similar fashion. The presence or absence of native kidneys was the most important factor determining hematocrit level and transfusion requirements in these patients, whether treated with testosterone or not. The mean hematocrit was lower and the transfusion requirements were higher in bilaterally nephrectomized patients. A significant increase in hematocrit occurred in testosterone treated nephric patients, but untreated nephric patients also had a significant rise. Important adverse side effects occurred with testosterone. Anephric patients did not increase hematocrit levels with or without testosterone.