49 individuals seropositive for human immunodeficiency virus (HIV) antibody were studied longitudinally for the relation between in-vitro properties of their sequential HIV isolates and clinical course before and after the development of AIDS. They were classified into three groups according to the syncytium-inducing capacity, replication rate, and host range of their HIV isolates. The most rapid progression to AIDS (median 15 months) and the lowest survival rate following AIDS diagnosis (median survival 12.5 months) were observed in individuals with high-replicating, syncytium-inducing HIV isolates, followed by individuals with high-replicating, non-syncytium-inducing isolates. In contrast, most individuals with low-replicating, non-syncytium-inducing HIV isolates remained symptom-free during the study period (median follow-up until AIDS diagnosis greater than 42 months), and the few individuals from this group in whom AIDS developed were still alive at the end of the study period (median survival greater than 34 months). In addition, AIDS patients from the three groups differed with respect to their symptoms. Zidovudine treatment in the symptom-free period seemed to delay the onset of AIDS in all risk groups, although stabilisation of CD4+ cell numbers was observed only in individuals with non-syncytium-inducing HIV variants.