We have conducted a longitudinal study on factors associated with candidal vaginal colonization, a precursor of vaginitis, in a cohort of HIV-infected women in Italy. All consecutive women attending a single, tertiary care clinical site were offered free screening for sexually transmitted infections and genital disorders every 6-12 months. Candidal vaginal colonization was defined as a positive culture for Candida spp. in an asymptomatic woman. From January 1998 to July 2002 we analysed 214 women. The baseline prevalence of candidal vaginal colonization was 16.8%. In the logistic regression analysis, the time since HIV infection > or =36 months (odds ratio [OR] = 0.18, 95% confidence interval [CI] 0.016-0.53, P = 0.002) and a plasma viral load > or =10,000 copies/mL (OR = 3.9, 95% CI 1.03-14.9, P = 0.045) were independently associated with candidal colonization. Among 130 women who were followed for a mean period of 24 months, the incidence of vaginal colonization was 10.7/100 women-years. In the Cox regression analysis, a CD4(+) T-lymphocytes count <100 cells/microL during the follow-up was associated with an increased risk of candidal vaginal colonization (OR = 4.45, C.I. = 1.20-16.81, P = 0.03). Risk of candidal vaginal colonization episodes in HIV-infected women significantly increase when CD4(+) T-lymphocytes are less than 100.