OBJECTIVEThe aim of this study was to elucidate whether subclinical nerve dysfunction as reflected by neurophysiological testing predicts the development of clinical neuropathy in patients with type 1 diabetes.RESEARCH DESIGN AND METHODSFifty-nine patients were studied twice with neurophysiological measurements at baseline and at follow-up. At baseline, patients were 15.5 3.22 years (range 7-22 years) of age, and duration of diabetes was 6.8 3.3 years. At follow-up, patients were 20-35 years of age, and disease duration was 20 +/- 5.3 years (range 10-31 years).RESULTSAt baseline, patients showed modestly reduced nerve conduction velocities and amplitudes compared with healthy subjects, but all were free of clinical neuropathy. At follow-up, clinical neuropathy was present in nine (15%) patients. These patients had a more pronounced reduction in peroneal motor nerve conduction velocity (MCV), median MCV, and sural sensory nerve action potential at baseline (P < 0.010-0.003). In simple logistic regression analyses, the predictor with the strongest association with clinical neuropathy was baseline HbA(1c) (R-2 = 48%, odds ratio 7.9, P < 0.002) followed by peroneal MCV at baseline (R-2 = 38%, odds ratio 0.6, P < 0.006). With the use of a stepwise forward analysis that included all predictors, first baseline HbA(1c) and then only peroneal MCV at baseline entered significantly (R-2 = 61%). Neuropathy impairment assessment showed a stronger correlation with baseline HbA(1c) ( = 0.40, P < 0.002) than with follow-up HbA(1c) ( = 0.034, P < 0.007).CONCLUSIONSEarly defects in nerve conduction velocity predict the development of diabetic neuropathy. However, the strongest predictor was HbA(1c) during the first years of the disease.