21 members of a large kinship with autosomal dominant CMT showing typical clinical findings were studied electroneurographically, with visual evoked potentials (VEP) and with blood group markers. In clinically affected members, the mean motor nerve conduction velocity (NCV) of the median nerve was found to be 17.5 m/s (SD 2.4). Contrary to previous genetic linkage studies in CMT families with comparable slow motor NCV, blood group typing in this family excluded close linkage of HMSN I to Duffy locus, which may indicate the existence of another subgroup in CMT neuropathy. Mean latencies of VEP, in both clinically affected and unaffected members, showed no pathological alterations when compared to normals. There was no correlation between NCV and P 100 latencies, but significant variation of P 100 latencies in families of twin brothers could be demonstrated. As already suggested by other authors, our findings may also indicate heterogeneity in this neuropathy.