Thirty-four consecutive patients with diabetes mellitus as a plausible cause of end-stage renal failure were investigated with regard to outcome and tobacco consumption. The mean duration of diabetes was 25.5 years. Thirty-one of the patients had used tobacco regularly for at least 5 years. For 28 patients it was possible to calculate the intake of tobacco as the daily mean and the total consumption (expressed as pack years). The time period (in years) between initiation of tobacco usage and onset of persistent proteinuria, as an early indicator of diabetic nephropathy, was shorter if the daily consumption of tobacco was higher (P less than 0.01) and the total consumption (pack years) in these patients was lower (P less than 0.05). There was an increased risk of early death (P less than 0.01) in patients who were currently smokers when they were uraemic (predialysis), on dialysis or had kidney grafts, compared to those who had never used tobacco or who were ex-smokers. In conclusion, it appears to be important for insulin-dependent diabetic patients to avoid tobacco usage and for current smokers to give up the habit in order to reduce the risk of diabetic nephropathy and early death.