This follow-up study examines whether the development of nerve dysfunction is retarded by tight metabolic control in patients with type I diabetes mellitus. Seventy-one patients and 115 age-matched healthy control subjects underwent studies of nerve conduction in peroneal and sural nerves. The presence of diabetes was associated with a reduction in peroneal motor nerve conduction velocity (MCV) by 5.9 m/s, sural sensory nerve conduction velocity (SCV) by 3.4 m/s, and sural sensory nerve action potential (SNAP) amplitude by 22%. Dysfunction in peroneal MCV, sural SCV, and sural SNAP were related to long-term poor metabolic control. Eleven of 12 patients with HbA1c <6.5% had normal nerve conduction or abnormality in only one nerve as compared to 2 of 15 patients with HbA1c >8.0%. It is concluded that tight long-term metabolic control (HbA1c <6.5%) can retard nerve dysfunction in patients with type I diabetes mellitus and a mean disease duration of 12 years.