Blood glucose values close to normal reduce the microvascular complications of insulin-dependent diabetes mellitus. The Stockholm study of this effect continued after the initial 7.5-year period in order to see what happened when intensively treated patients were left to control their own treatment while treatment was intensified in the control group. Forty-three patients with insulin-dependent diabetes randomised to intensified conventional treatment (ICT) and 48 patients randomised to standard treatment (ST) were followed-up for 10 years. Vascular complications, treatment side-effects and well-being were studied. Risk factors for complications were sought. HbA1c (normal range 3.9-5.7%) was reduced from 9.5 +/- 1.4% (mean +/- SD) in the ICT group and 9.4 +/- 1.2% in the ST group to a mean (during 10 years) of 7.2 +/- 0.6% and 8.3 +/- 1.0%, respectively (p < 0.001). Serious retinopathy (63 vs 33%, p = 0.003), nephropathy (26 vs 7%, p = 0.012) and symptoms of neuropathy (32 vs 14%, p = 0.041) were more common in the ST group after 10 years. HbA1c and age were the only risk factors for complications. Self-reported well-being increased to a greater degree and severe hypoglycaemia was more common in the ICT group. Cognitive function after 10 years was similar in both treatment groups, and was not related to the number of severe hypoglycaemic episodes. Intensified insulin treatment leads to reduced long-term complications and increased well-being without causing undue side-effects.