We investigated 51 patients with multifocal neuropathies for the deletion of chromosome 17p11.2 described in families with hereditary neuropathy with liability to pressure palsies (HNPP). The deletion was detected in 24 patients, including 19 patients from 14 of 15 families in whom HNPP had been considered likely on clinical, neurophysiological, and/or pathological grounds. One patient with a deletion had rather unusual clinical features for HNPP, presenting with a progressive scapuloperoneal syndrome. Overall, 7 (37%) of the 19 index patients with the deletion had no affected relatives, and less than half had evidence of a generalized neuropathy on examination. Peripheral nerve lesions were related to pressure in only 15 (62%) of the patients with the deletion. Nerve conduction studies in 23 of 25 patients and relatives studied showed a fairly uniform pattern of moderate prolongation of distal sensory and motor latencies and slowing of conduction velocities, and variable reduction of sensory or evoked muscle action potential amplitudes. The patients investigated who did not have a deletion of 17p11.2 were heterogeneous and included those with recurrent and/or familial neuralgic amyotrophy, two or more peripheral nerve lesions at common sites of entrapment, or a patchy axonal neuropathy of unknown etiology. In 1 patient a diagnosis of HNPP remains most likely. DNA analysis for the deletion of 17p11.2 is clearly useful in establishing the diagnosis of HNPP, which should be considered regardless of family history or clinical evidence of a generalized neuropathy, and in patients with multifocal neuropathies that do not conform to the classic clinical practice of HNPP.