Nerve and muscle biopsies were performed on 20 patients with HIV infection and peripheral neuropathy. Nine patients had distal symmetrical peripheral neuropathy (DSPN) (six ARC and three AIDS), six had inflammatory demyelinating polyneuropathy (IDP) (three ARC, one AIDS, and two otherwise asymptomatic patients), one had mononeuropathy multiplex (MM) (AIDS), 1 had mononeuropathy (ARC), one had meningoradiculitis (AIDS), and two had areflexia-associated lymphocytic meningitides (ARC), DSPN exhibited axonal degeneration in four of nine cases and was associated with segmental demyelination in five of nine cases. IDP exhibited segmental demyelination associated with axonal degeneration in four of six cases. Demyelination was more frequent in asymptomatic patients (2 of 2 cases) and in ARC (7 of 12 cases), whereas axonal degeneration was predominant in AIDS (6 of 6 cases). Mononuclear cell infiltration was seen in 1 of 2 asymptomatic patients and in 11 of 12 ARC patients but was exceptionally found in AIDS (1 of 6 cases). Involvement of the walls of small vessels, mostly venules ("subacute microvasculitis"), was found in 1 of 2 asymptomatic patients, in 8 of 12 ARC patients, and never in AIDS. The polyclonal mononuclear cell population was composed mainly of Leu 2 (T8) positive cells in seven cases of ARC. No virions were seen in electron microscopy. HIV was isolated in two cases from the CSF or the nerve biopsy.