Haematological changes after renal transplantation in 76 patients were reviewed and the differences observed between patients treated with cyclosporin-A and prednisolone and those treated with azathioprine and prednisolone were compared. Erythrocytosis defined as haemoglobin concentration equal to or exceeding 17 g/dl occurred in 25% of patients treated with cyclosporin-A and in 11.4% of patients treated with azathioprine. Only one patient, who received cyclosporin-A, had clinical evidence of thrombosis. Eight patients treated with cyclosporin-A and five treated with azathioprine had therapeutic venesections. There was no difference in the incidence of putative risk factors for post-transplant erythrocytosis between the two groups.