Hypertension is a major complication in kidney transplantation and contributes to the high cardiovascular mortality of renal transplanted recipients. The aim of the present study was to evaluate the therapeutic effect of phlebotomy on blood pressure in posttransplant hypertension associated with erythrocytosis. In 12 renal transplanted patients (7 male, 5 female, aged 29-52 years) with erythrocytosis (defined by hematocrit > 52% or hemoglobin > 170 g/l), a 24-hour-monitoring of blood-pressure and heart rate (SpaceLabs SL90207) was performed before, 2 and 6 weeks after phlebotomy. Patients with iron-deficiency and/or transplant rejection were excluded from the study. Ten of 12 patients were on antihypertensive treatment before phlebotomy. Phlebotomy (500 ml) was repeated three times on average within the first two weeks, until hematocrit decreased below 45%. The phlebotomy therapy lowered the hematocrit after two weeks from 54.8 +/- 2.8% to 44.3 +/- 4.2% and 43.0 +/- 5.6% after six weeks. Before phlebotomy, the blood pressure was systolic 153.2 +/- 15.1 mmHg and diastolic 95.2 +/- 9.5 mmHg. After repeated phlebotomy, there was a significant decrease of blood pressure to systolic 139.0 +/- 14.1 and diastolic 85.3 +/- 8.2 mmHg (p < 0.01). Without change of hematocrit and hemoglobin, there was no further change of blood pressure after six weeks (systolic 140.1 +/- 9.9 mmHg, diastolic 86.3 +/- 9.5 mmHg). The heart rate did not change significantly during the therapy. The antihypertensive treatment could be reduced in most of the patients. The present study demonstrates the therapeutic effect of phlebotomy in posttransplant hypertension associated with erythrocytosis.(ABSTRACT TRUNCATED AT 250 WORDS)