Urinary neopterin, a sensitive marker for activation of cell mediated immunity, is compared to clinical and laboratory data in 5 patients with AIDS, in 15 patients with ARC and in 40 male subjects without AIDS-related clinical signs attending an AIDS outpatient clinic. The sensitivity of neopterin for AIDS-related diseases was higher than that of the CD4+/CD8+ subset ratio. The differences in neopterin levels between the controls, ARC and AIDS patients were found to be more significant than the T-cell subset data. The observation that the 3 AIDS patients with the highest neopterin levels have died whereas the two others with the lower levels are still alive also underlines the prognostic potential of urinary neopterin level determinations. Measurement of urinary neopterin is thus recommended as an additional criterion for monitoring ARC and AIDS patients.