Systemic hypertension as assessed by causal blood pressure measurements is a frequently reported side-effect of recombinant human erythropoietin (rHuEpo) treatment. We investigated the effect of rHuEpo treatment on the 24-h ambulatory blood pressure and heart rate profiles of 13 chronic haemodialysis patients. After 3-4 months of rHuEpo therapy it was found that the mean haematocrit had increased from 24.5 +/- 1.0% to 32.0 +/- 1.1% (P less than 0.005), while body-weight and control of uraemia as assessed by routine laboratory data remained unchanged. Despite gradual and incomplete correction of anaemia by use of low doses of rHuEpo, increases in the ambulatory systolic and diastolic blood pressure were found. The greatest increases affected day-time systolic blood pressure and night-time diastolic blood pressure, and these increases were significant (P less than 0.05). As a result, pulse pressure increased during day-time (P less than 0.05) while the night-time decline in diastolic blood pressure disappeared. An increase in peripheral resistance after partial correction of renal anaemia might explain these observations. rHuEpo therapy increased the percentage of abnormal ambulatory blood pressure measurements (defined as systolic blood pressure greater than 140 mmHg and/or diastolic blood pressure greater than 90 mmHg) from 33% to 52% (P less than 0.05) while in contrast, mean casual prehaemodialytic and posthaemodialytic blood pressure values remained unchanged. We conclude that changes in 24-h blood pressure profiles should be carefully assessed by ambulatory blood pressure monitoring in haemodialysis patients treated with rHuEpo, since these changes are likely to be missed when only causal blood pressures are measured.