To investigate diabetic polyneuropathy, we measured peroneal motor conduction velocity, sural sensory nerve conduction velocity and vibratory sense threshold (biothesiometry) in 28 children and adolescents with insulin-dependent diabetes (type 1), and in 28 age- and sex-matched, normal controls. Age varied from 8 to 19 yr (mean +/- SD = 13.04 +/- 2.61); age at the onset of diabetes from 9 months to 12 yr (4.53 +/- 2.42 yr); and the duration of diabetes from 5 to 16 yr (8.48 +/- 2.98). Eight patients (28%) fulfilled the minimal criteria for the diagnosis of polyneuropathy. Four of these patients showed symptoms while three had clinical signs of neuropathy. Eight patients had abnormal, sural sensory nerve conduction velocities. The presence of polyneuropathy did not correlate with the duration of diabetes or degree of metabolic control of diabetes. The prevalence of microvascular complications (microalbuminuria and retinopathy) was 32%. The presence of microvascular complications did not correlate with metabolic control but did with the duration of diabetes. The relationship between polyneuropathy and microvascular complications was 34%.