We examined the emergence of CXCR4 (i.e., X4) tropism in 67 male human immunodeficiency virus type 1 (HIV-1) seroconverters from the Multicenter AIDS Cohort Study (MACS) who were selected to reflect the full spectrum of rates of HIV-1 disease progression. A mean of 10 serial samples per donor were evaluated by a laboratory-validated, commercially available assay to determine phenotypic coreceptor use. A total of 52% of men had dual- or mixed-tropic HIV-1 detected at 1 or more of the time points tested. Use of X4 by HIV-1 was detected more frequently among men who developed AIDS (defined as a CD4(+) T cell count of <200 cells/muL and/or an AIDS-defining illness) < or =11 years after seroconversion than among those who did not (P = .005), as well as among men who exhibited a total T cell count decline (i.e., a CD3(+) inflection point), compared with those who did not (P = .03). For men in whom both X4 virus and an inflection point were detected, emergence of X4 virus preceded the inflection point by a median of 0.83 years. The median CD4(+) T cell count at first detection of X4 viruses before the onset of AIDS was 475 cells/microL. We conclude that HIV-1 variants that used X4 frequently emerged at high CD4(+) T cell counts and may contribute to the decrease in T cell numbers during late HIV-1 infection.