Patients with insulin-dependent diabetes mellitus (IDDM), non-proliferative retinopathy and unsatisfactory blood glucose control were randomized to intensified conventional treatment (ICT, 48 patients) or regular treatment (RT, 54 patients) for a 5-year study. After 18 months the glycosylated hemoglobin (HbA1c) was reduced in both groups, but significantly more in the ICT group (p = 0.00005). Thirty of the RT patients and 16 from the ICT group deteriorated as to retinopathy (p = 0.024). Microalbuminuria appeared more often in the RT patients (p = 0.023), and nerve conduction velocities were significantly reduced only in the RT group (p between 0.0005 and 0.047). Serious hypoglycemia was more common in the ICT patients (p = 0.003). The progression of diabetic late complications was thus slowed down by intensified treatment, but at the price of an increased frequency of serious hypoglycemia.