Peroneal motor and sural sensory conduction velocities (MNCVs/SNCVs), somatosensory evoked potentials to median nerve stimulation (MN-SEPs) and motor evoked potentials (MEPs) to transcranial stimulation were examined in 138 HIV-infected patients (in the different stages of the disease), 20 seronegative intravenous drug abusers (IVDAs), and 20 healthy subjects. Findings of peroneal MNCV slowing in patients ranged from 16% (asymptomatic HIV patients) to 63% (AIDS) and of sural SNCV slowing from 13% to 40%. Altered MN-SEPs ranged from 10% to 30%, and MEPs ranged from 44% to 72%, mostly due to a prolongation of the central motor conduction time (CMCT). All seronegative IVDAs showed patterns within the normal range. Electrophysiological techniques were helpful in demonstrating early and subclinical alterations in HIV patients.