In a case control study 192 cigarette-smoking patients with Type 1 (insulin-dependent) diabetes were compared with 192 non-cigarette-smoking patients pair-matched for sex (90 females), duration of diabetes (mean 14 years), and age (mean 32 years). Macroproteinuria was found in 19.3% of the smoking and in 8.3% of the non-smoking patients (p less than 0.001). Proliferative retinopathy was present in 12.5% of the smoking and in 6.8% of the non-smoking patients (p less than 0.025). The percentages of patients with normal proteinuria or without retinopathy were comparable between the two groups. In addition, glycosylated haemoglobin values and the prevalence of hypertension were similar between smoking and non-smoking patients. Thus, cigarette-smoking appears to be a risk factor for the progression of incipient to overt nephropathy and of background to proliferative retinopathy in Type 1 diabetes.