Hyperparathyroidism has been implicated as a cause for resistance to erythropoietin therapy, based on a limited number of studies. The authors retrospectively surveyed the effects of parathyroidectomy on hematocrit and erythropoietin dose in patients with end-stage renal disease. The study group included 10 patients (4 men, 6 women) on long-term hemodialysis, 9 of whom were receiving erythropoietin intravenously during dialysis. Parathyroidectomy resulted in a decrease in mean parathormone levels from 15 times normal preoperatively to 2.3 times normal at follow-up. The hematocrit values increased, and the weekly erythropoietin dose decreased significantly when values 1 month before surgery were compared with those 6 months after surgery. From this, the authors conclude that secondary hyperparathyroidism is a cause of worsening anemia and increased erythropoietin requirements in patients on hemodialysis, and that parathyroidectomy is effective in reversing these abnormalities.