Motor and sensory nerve conduction velocities (MNCV, SNCV), beat-to-beat variation (BBV) at rest, speed of pupillary dilation (SPD), and pupillary latency time (PLT) were measured in 32 patients aged 12-36 years after 19 +/- 2 (mean +/- SEM) days and again after 3, 12, and 24 months of insulin treatment. Moreover, BBV under deep respiration was determined after 12 and 24 months, and thermal discrimination thresholds (TDT) as well as pain and vibration perception thresholds (PPT; VPT) were evaluated after 24 months. Mean HbA1 levels during months 3-24 within the normal range (7.2 +/- 0.2%; mean +/- SEM) were observed in 20 patients (group 1), while in 12 patients (group 2) mean HbA1 of months 3-24 was elevated (10.1 +/- 0.4%). There were no significant differences between both groups with regard to the nerve function tests at baseline and after 3 months. After 12 months mean median MNCV and median, ulnar, and sural SNCV were significantly lower in group 2 than in group 1 (p less than 0.05). After 24 months mean median MNCV, peroneal MNCV, median SNCV, and sural SNCV as well as both BBV tests and PLT were significantly impaired in group 2 as compared to group 1 (p less than 0.05). In addition, mean malleolar VPT and PPT to heat and cold stimuli on the thenar and the foot were significantly elevated in group 2 as compared to group 1 (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)