The reliability of laboratory parameters for the prediction of impending manifestation of AIDS in HIV-1 infected patients was investigated. Over a mean observation period of approximately 1 year (5 to 33 months) 56 patients were studied. Eleven patients progressed to AIDS (group 1) and 45 remained free of AIDS-defining disease (group 2). Eight candidate-predictor variables were measured on each patient at the beginning of the observation period: neopterin, hemoglobin, lymphocytes, CD4 counts, CD8 counts, platelets, gammaglobulins, and erythrocyte sedimentation rate. In a linear discrimination analysis, a stepwise entry of these parameters into the discriminant function was effected according to which variable added most to the separation into groups 1 and 2. Neopterin proved to be most discriminating with no improvement of separation when adding further variables. Thus, a final allocation rule was computed using neopterin alone. This rule assigns neopterin levels above 21.9 nmol/l to group 1 and levels below to group 2. We conclude, tentatively, that in HIV-infected patients neopterin levels of 22 nmol/l and over may indicate the manifestation of AIDS in the year to come.