Human immunodeficiency virus type 1 (HIV-1) appears to adversely affect hepatitis C, but whether hepatitis C virus (HCV) has a reciprocal effect on HIV-1 infection remains a point of controversy. In a multivariate analysis of a cohort of 5832 individuals, we found that individuals coinfected with HCV and HIV-1 (prevalence of coinfection, 5.8%) had a CD4+ cell count that decreased at a rate similar to that for individuals infected with HIV-1 alone. However, coinfection was associated with a statistically significant increased likelihood of onset of an acquired immunodeficiency syndromedefining illness or developing a CD4+ cell count of <200 cells/mm3, compared with infection with HIV-1 alone (hazard ratio, 1.52; 95% confidence interval, 1.072.17). Patients who were naive to highly active antiretroviral therapy were significantly less likely to progress to either end point, because of their higher CD4+ cell counts. In conclusion, there was an increased number of adverse events in coinfected individuals, compared with individuals infected with HIV-1 alone.