The effect of early antihypertensive treatment on survival of patients with diabetic nephropathy was evaluated by studying two cohorts of Type 1 (insulin-dependent) diabetic patients developing persistent proteinuria in I: 1957-1973 (late treatment group n = 49) and II: 1979-1983 (early treatment group n = 71). At onset of nephropathy, the two cohorts were comparable with regard to age (29(8) vs 30(8) years, mean (SD], duration of diabetes (16(6) vs 18(7) years), blood pressure (132(16)/85(11) vs 134(16)/86(8) mm Hg), proteinuria (0.8(0.5-1.2) vs 0.8(0.6-1.2) g x 24 h-1, median (quartiles] and serum creatinine (87(14) vs (85(16) mumol x 1(-1]. The patients were followed frequently at the outpatients' clinic until death or for a median duration of 8 years. In the first cohort antihypertensive treatment was seldom used, whereas, in the second cohort antihypertensive treatment was started when blood pressure reached 144(18)/93(7) mm Hg. The probability of survival with a functioning kidney for more than 8 years was 48% in the first cohort and 87% in the second cohort, p less than 0.001. The improvement of survival was due mainly to a decreased mortality from uraemia. Early antihypertensive treatment is the most likely explanation for this improvement.