SummaryWe have investigated the effect of long-term strict glycaemic control on peripheral and autonomic nerve function in 45 IDDM patients (age 18-42 years, diabetes duration 7-23 years) without clinical signs of neuropathy or other neurological disease. They were randomly assigned to treatment either with continuous insulin infusion, multiple injections (4-6 times daily), or conventional treatment (twice daily) for 4 years and followed prospectively for 8 years. Motor and sensory nerve conduction velocities were measured at the start and after 8 years. Autonomic nerve function tests were performed only once, after 8 years. A significant reduction of nerve conduction velocity was observed during 8 years in patients with mean HbA1 more than 10% (n = 12, group mean 10.9%, range 10.1-13.2%) compared to patients with HbAI less than 10% (n = 33, group mean 9.0 %, range 7.5-9.9 %). Change of motor nerve conduction velocity in the peroneal nerve was: -4.8 + 4.9 (SD) vs -2.2 + 5.3 m/s (p < 0.01). Change of motor nerve conduction velocity in the posterior tibial nerve was: -6.8 + 5.7 vs-3.9 +_ 5.1 m/s (p < 0.05). No significant changes were observed in the ulnar nerve. Change of sensoric nerve conduction velocity in the sural nerve was: -8.9 + 8.0 vs -4.6 + 5.3 m/s (p < 0.05). Multiple regression analysis showed that a change in HbA1 of 1% resulted in a 1.3 m/s change in nerve conduction velocity during 8 years. A significantly lowered heart-rate variation during deep breathing (p < 0.05) and heart-rate response to standing (p < 0.01) was found in patients with HbA1 more than 10 % compared to patients with HbA1 less than 10 %. This study confirms that the long-term lowering of blood glucose retards the deterioration in nerve conduction velocity observed in the diabetic nerve. [Diabetologia (1994) 37: 579-584]