In order to assess the actual value of the in vitro tests of cellular immunity performed to provide diagnostic evidence for acquired immune deficiency syndrome (AIDS) or related syndromes, we have evaluated T-lymphocyte subsets and in vitro proliferative responses. The respective profiles of 44 symptom-free homosexual men, 30 patients with AIDS and 42 patients with lymphadenopathy were compared with each other and with profiles of blood bank volunteers, using stepwise discriminant analysis. At first investigation, control homosexuals, AIDS patients and lymphadenopathy patients displayed varying degrees of the same type of immunological abnormalities. The percentage of OKT4+ cells was the most discriminant variable between blood donors and control homosexuals, AIDS or lymphadenopathy patients. The latter differed from homosexuals by a higher percent of OKT8+ cells and from AIDS patients by a lesser reduction in T-lymphocyte counts. OKT4+ cell counts differentiated AIDS patients from control homosexuals. However, none of these major discriminant variables could classify correctly more than 70% of subjects in the homosexual and in the two patient groups. Thus, a single investigation of T-lymphocyte immune parameters does not provide substantial diagnostic and prognostic evidence.