Twenty-seven patients with renal failure (16 on CAPD and 11 predialysis) were treated with erythropoietin. At 12 weeks, the mean haemoglobin concentration (+/- SEM) in the CAPD patients had increased from 7.07 +/- 0.20 to 10.88 +/- 0.45 g/dl (two-tailed paired t test, P < 0.0001) and in the predialysis patients from 6.90 +/- 0.35 to 10.05 +/- 0.47 g/dl (P < 0.0001). Predialysis patients were taking more antihypertensive medication at baseline. No increase was required in either group after erythropoietin; there was no change in blood pressure in the CAPD patients, though in the predialysis patients the systolic blood pressure rose slightly from 132 to 146 mmHg (P = 0.029) and the mean blood pressure from 95 to 103 mmHg (P = 0.028). In 12 patients (6 on CAPD and 6 predialysis) the red cell volume, plasma volume, and total blood volume were measured before and after treatment. In the CAPD patients there was a marked expansion of the red cell volume from 912 +/- 127 to 1471 +/- 222 ml (P = 0.004) and a concomitant contraction of the plasma volume from 3932 +/- 250 to 3178 +/- 326 ml (P = 0.005), leaving the blood volume unchanged from 4843 +/- 352 to 4649 +/- 503 ml. Predialysis patients had a similar expansion of the red cell volume from 733 +/- 59 to 1304 +/- 161 ml (P = 0.017) but no contraction of the plasma volume (from 3417 +/- 354 to 3314 +/- 260 ml), so that the blood volume tended to expand from 4149 +/- 347 to 4618 +/- 414 ml (P = 0.053).(ABSTRACT TRUNCATED AT 250 WORDS)