Patients with orthostatic hypotension caused by autonomic neuropathy frequently have a decreased red-cell mass. This would be expected to compromise their effective circulating blood volume and aggravate the orthostatic hypotension. We studied the effect of increasing the red-cell mass with erythropoietin, given subcutaneously in a dose of 50 U per kilogram of body weight three times a week for 6 to 10 weeks to eight patients with orthostatic hypotension--four men, one teenage boy, and three women (age range, 17 to 68 years). Four patients had type I diabetes mellitus and autonomic neuropathy, three patients had pure autonomic failure, and one patient had sympathotonic orthostatic hypotension. Seven patients received fludrocortisone (0.1 or 0.2 mg per day) before, during, and after the trial of erythropoietin. The red-cell volume, plasma volume, and hemodynamic response to orthostatic stress were measured before and after therapy.Erythropoietin increased the mean (+/- SD) hematocrit from 0.34 +/- 0.04 to 0.45 +/- 0.04 (P < 0.005) and increased the red-cell volume from 16.8 +/- 3.9 to 25.3 +/- 3.1 ml per kilogram (P < 0.005), but had no effect on plasma volume. The systolic blood pressure increased from 81 +/- 11 to 100 +/- 24 mm Hg (P < 0.01) and the diastolic blood pressure increased from 46 +/- 10 to 63 +/- 18 mm Hg (P < 0.01) while the patients were standing. The average systolic and diastolic blood pressure while the patients were supine did not increase significantly, although hypertension in the supine position developed in three patients. Orthostatic dizziness improved during treatment in six of the eight patients.In patients with orthostatic hypotension, increasing the red-cell volume with erythropoietin elevates blood pressure while standing. Possible long-term adverse effects are not known.