Whole blood granulocyte aggregation in vitro was measured in 17 haemodialysis (HD) patients, 18 continuous ambulatory peritoneal dialysis (CAPD) patients, and 30 age-matched normal controls. Granulocyte aggregation was significantly increased in dialysis-dependent patients compared to controls (P less than 0.001) and was statistically significantly higher in CAPD patients compared to HD patients (P less than 0.01). [CAPD patients, median (range): 56.5 (44-71)%; HD patients: 53.0 (32-67)%; Controls; 42.5 (33-56)%.] Since granulocyte hyperaggregability has been linked with the pathogenesis of ischaemic vascular disease, abnormal granulocyte activity in dialysis-dependent patients may be of many factors contributing to their increased cardiovascular morbidity.