In order to assess the possible progression of neurological abnormalities over time and the value of visual evoked potential alterations in predicting stability and severity of diabetes-related optic pathway disease, a longitudinal study in non-insulin-dependent diabetic patients was performed. Neurological examination, visual evoked potentials with pattern reversal, motor and sensory nerve conduction velocities and metabolic control were studied in 18 non-insulin-dependent diabetic patients and in 35 normal control subjects at baseline and again after 4.6 +/- 0.8 years (range 4-6). At the first recording the peak P100 wave latencies were significantly delayed in the diabetic patients compared with the control subjects; signs of peripheral neuropathy were detected in five patients, clinical in three and in two there was only neurophysiological alteration without clinical signs. The second recording revealed no significant alterations of P100 latencies in patients compared with baseline, but the number with clinical signs and/or neurophysiological alterations with no clinical signs of peripheral neurological disease was increased to seven. In conclusion, we observed that visual evoked potential alterations were stable over time whereas peripheral neurological disease progressed and correlated positively with metabolic control.