Ninety-six patients with insulin-dependent diabetes mellitus (IDDM) and non-proliferative retinopathy were randomized to intensified conventional treatment (ICT) (n = 44) or regular treatment (RT) (n = 52), and followed up for 5 years. HbA1c decreased from 9.5 +/- 0.2% (mean value +/- SEM) to 7.2 +/- 0.1% in the ICT group, and from 9.4 +/- 0.2% to 8.7 +/- 0.1% in the RT group (difference between the groups, P less than 0.001). Retinopathy increased in both groups (P less than 0.001), but after 5 years it was worse in the RT group (P less than 0.05). The urinary albumin excretion rate was higher in the RT group than in the ICT group after 5 years (239.9 +/- 129.7 micrograms min-1 vs. 46.0 +/- 26.1 micrograms min-1, P less than 0.05). Eight RT patients developed manifest nephropathy, compared with none in the ICT group (P less than 0.01). After 5 years the conduction velocities of the sural (P less than 0.05), peroneal (P less than 0.01) and tibial (P less than 0.001) nerves were lower in the RT group. The respiratory sinus arrhythmia was 12.1 +/- 1.2 beats min-1 in the RT group and 16.7 +/- 1.4 beats min-1 in the ICT group at the end of the study (P less than 0.01). The increases in retinopathy (P less than 0.01), nephropathy (P less than 0.01) and neuropathy (P less than 0.001) were all related to the mean HbA1c value during the study. Smoking habits only influenced the progression of retinopathy (P less than 0.05). Serious hypoglycaemia occurred in 34 ICT patients and 29 RT patients (242 and 98 episodes, respectively) (P less than 0.05). Whereas weight was stable in the RT group, the body mass index increased by 5.8% in the ICT group (P less than 0.01). In conclusion, microvascular complications of diabetes were retarded by intensified conventional insulin treatment. However, such treatment increased the frequency of serious hypoglycaemia, and led to an increase in body weight.