To assess subclinical involvement of the peripheral nervous system and its relationship to the immunological status of human immunodeficiency virus (HIV)-infected patients, we prospectively studied the peripheral nerve conductions and the subsets of peripheral blood lymphocytes of HIV patients. Fifty-seven patients, aged 20-54 years, 28 homosexuals and 29 heterosexuals, classified as CDC II-III (40 patients) and CDC IV (17 patients) were studied. No neurological symptoms or signs were present on bedside examination or questioning for all of the CDC II-III patients. For each conduction measured (H-reflex, sural and sciatic velocities, sural amplitude), the geometric mean was below normal values (p less than 0.05). Among the 57 HIV-infected patients, 20 (35%) had a significant decrease of at least one electrophysiologic measurement: 10/17 CDC IV patients vs. 10/40 CDC II-III patients (p less than 0.05). Two or more simultaneous abnormalities were more often observed in the CDC IV than in the CDC II-III group (9/10 vs. 4/10) (p less than 0.01). In the CDC II-III group, patients with subclinical neuropathies had higher T-helper and lower T-suppressor cell counts and higher T-helper/T-suppressor ratios than random patients without any neurologic abnormalities (1,057 vs. 657 cells/microliters, p less than 0.05, and 757 vs. 1,304 cells/microliters, p less than 0.01, 1.55 vs. 0.68, p less than 0.01, respectively). We conclude that (a) the peripheral nervous system is precociously and subclinically involved in the HIV disease; (b) the patients with infraclinical neuropathies have a significantly higher T-helper cell count than those without any neurologic involvement. The mechanism and the prognostic value of these two findings remain to be more precisely examined, and further studies are required.