Spontaneous renal allograft rupture occurring within 14 days of transplantation occurred in 15 patients from 791 consecutive transplants. In each of eight patients treated with azathioprine and prednisolone there was pathological evidence of rejection and only two patients had thrombosis of the renal vein. Of the seven cases occurring in patients treated with triple therapy regimen (low dose cyclosporin, prednisolone and azathioprine), histological evidence of rejection was present in only three cases, but renal vein thrombosis was found in all seven. Spontaneous rupture of a transplanted kidney, a relatively uncommon complication, is more likely to be due to renal vein thrombosis than to rejection in the cyclosporin era.