Human immunodeficiency virus type 1 (HIV-1)-infected individuals develop a genetically diverse virus population over time, but often only a limited number of viral variants are transmitted from a chronic carrier to a newly infected person. Interestingly, many women but few men are infected by multiple HIV-1 variants from a single partner. To determine whether the complexity of the infecting virus population influences clinical outcome, we examined viral diversity in the HIV-1 envelope sequences present at primary infection in 156 women from Kenya for whom we had follow-up data on viral RNA levels and CD4 T-cell counts. Eighty-nine women had multiple viral genotypes, while 67 women had a single genotype at primary infection. Women who acquired multiple viral genotypes had a significantly higher viral load (median, 4.84 versus 4.64 log10 copies/ml, P = 0.04) and a significantly lower CD4+-T-cell count (median, 416 versus 617 cells/mm3, P = 0.01) 4 to 24 months after infection compared to women who were infected with a single viral genotype. These studies suggest that early HIV-1 genetic diversity is linked to faster disease progression.