We studied the prognostic value on the decrease of CD4 lymphocyte count of anti-p24 antibody (ab) titer and compared this value with that of polyclonal and monoclonal p24 (ag) titer before and after immune complex dissociation (ICD); 53 human immunodeficiency virus (HIV)-infected patients having CD4+ counts above 400/mm3 when first examined were followed up over a 3-year period including at least four visits; HIV disease progressors (n = 18) were defined as having CD4+ counts below 200/mm3 and non-progressors (n = 35) as having CD4+ counts still above 400/mm3 at the end of follow-up. Sera were collected at each visit and assayed for p24 ag with and without ICD and for anti-p24 ab titer. The mean anti-p24 ab titer of progressors and of non-progressors at entry was significantly different. A threshold of anti-p24 ab titer indicating a HIV progression was determined at 1/300. The proportion of individuals with an anti-p24 ab titer lower than 1/300 at least once during the study period was 34% in non-progressors and 94% in progressors. The difference between progressors and non-progressors at entry was significant with monoclonal p24 ag without ICD and more significant with monoclonal p24 ag after ICD. The marker having the highest predictive value was the anti-p24 ab titer, then monoclonal p24 ag with ICD, then polyclonal p24 ag with ICD. Anti-p24 ab is an earlier and stronger marker of the decrease of CD4 lymphocyte count than p24 ag even after ICD.