Anemia is common in patients with chronic renal insufficiency and secondary hyperparathyroidism. Erythropoietin therapy is effective, but the dose required varies greatly. One possible determinant of the efficacy of erythropoietin therapy is the extent of marrow fibrosis caused by hyperparathyroidism. We examined the relation between the erythropoietic response to erythropoietin and hyperparathyroidism in a cross-sectional study of 18 patients undergoing hemodialysis who had received erythropoietin therapy for one to three years. In 7 patients (the poor-response group), the dose of intravenous erythropoietin needed to maintain a mean (+/- SD) target hematocrit of 35 +/- 3 percent was > 100 units per kilogram of body weight three times a week, and in 11 patients (the good-response group) it was < or = 100 units per kilogram. In all patients, indexes of the adequacy of dialysis and the extent of hyperparathyroidism and aluminum toxicity were determined monthly, and bone histomorphometry was performed. The mean (+/- SD) dose of erythropoietin required to maintain the target hematocrit was 174 +/- 33 units per kilogram three times a week in the poor-response group and 56 +/- 18 units per kilogram in the good-response group. The mean ages, duration and adequacy of dialysis, increment in hematocrit, iron requirements, and serum concentrations of calcium, phosphate, and aluminum were similar in the two groups. The percentages of osteoid volume and surface, the osteoid thickness, and the stainable aluminum content of bone were similar in the two groups. In contrast, the mean serum parathyroid hormone concentration, the percentages of osteoclastic and eroded bone surfaces, and the degree of marrow fibrosis were greater in the poor-response group than in the good-response group (P = 0.03, P = 0.04, P = 0.009, and P = 0.009, respectively). In patients with uremia, the dose of erythropoietin needed to achieve an adequate hematocrit response may depend on the severity of secondary hyperparathyroidism and the extent of bone marrow fibrosis.