Among the HIV-1 seropositive subjects detected through blood donation in Paris area who every six months voluntarily went through a thorough clinical and biological follow-up in the Institut National de Transfusion Sanguine, a cohort of 77 subjects had completed at least four biannual controls by september 1988. Upon inclusion in the study, all subjects were CDC stage II or III, none had received any treatment. Biological parameters in entrance in the study were assessed in a attempt to forecast the CD4 lymphocyte count decrease during the 18 months follow-up time. Multivariate forward stepwise analysis indicated that only p24 ag level, IgA level, and partially neopterin or Beta-2-Microglobulin levels were independently predictive of CD4 count at the end of the follow-up and of the CD4 loss at the same time. The prediction by these biological parameters is a rather poor one, 39% of the variation of CD4 lymphocyte count or 30% of the variation of CD4 loss after 18 months of the cohort follow-up being explained. In conclusion, HIV-infection seems to possess an intrinsic evolution which escapes our surveillance by biological parameters.