In 7 long-term survivors (LTS) and 8 progressors, all carrying solely non-syncytium-inducing variants, a possible correlation between in vitro virus replicative capacity, virus load, and clinical course of human immunodeficiency virus type 1 (HIV-1) infection was analyzed. Late in infection, 3 LTS and 7 progressors had a high virus load, which coincided with the presence of rapid-replicating viruses. In contrast to progressors, LTS maintained relatively high and stable CD4 T cell counts. Four LTS persistently had relatively slow-replicating viruses and a low virus load, even after 6.6-9 years of seropositive follow-up. All virus isolates from 1 of these LTS had a 4-aa deletion in nef. These results suggest a correlation between the in vitro replicative capacity of non-syncytiuminducing HIV-1 variants and virus load. The presence of HIV-1 variants with relatively low replicative capacity throughout infection may have contributed to the beneficial clinical course in half of the LTS in this study.