Ninety-seven patients with insulin dependent diabetes mellitus (IDDM) were randomized to intensified conventional treatment (ICT, n = 44) or regular treatment (RT, n = 53). The mean HbA1c level (+/- SEM) was reduced from 9.5 +/- 0.2% to 7.4 +/- 0.1% in the ICT group (P less than 0.001), and from 9.4 +/- 0.2% to 9.0 +/- 0.2% (P less than 0.01) in the RT group. The difference between the groups was significant (P less than 0.001). During a period of 3 years, 57% of the ICT patients (95% confidence interval 44-73%) and 23% of the RT patients (95% CI, 11-34%) (P less than 0.001) had at least one episode of serious hypoglycaemia, with the need for third-party assistance or resulting in coma. Eighteen of the 32 ICT patients who initially had adrenergic symptoms during hypoglycaemia changed to predominantly neuroglycopenic symptoms. This was the case with only 8 of 38 RT patients (P less than 0.01). The change in symptoms was related to the increased frequency of serious hypoglycaemia, but neither symptoms nor frequency of hypoglycaemia bor any relationship to insulin dose, body mass index, duration of diabetes or autonomic nerve function. The results of several neuropsychological tests did not differ between the groups at baseline, and did not change during the study. There were no signs of deteriorating cognitive function in the patients with serious hypoglycaemic episodes.