BACKGROUND: Our objective was to examine whether virological control during the first 6-18 months after HAART initiation is a predictor for viral suppression, CD4+ cell count increase, and mortality in human immunodeficiency virus (HIV)-infected patients 18-90 months after initiation of highly active antiretroviral therapy (HAART). METHODS: We conducted a population-based observational cohort study in Denmark. Patients were divided into 3 groups, according to the proportion of time each patient had a detectable HIV RNA load (i.e., > or = 400 copies/mL) during the 6-18 months after HAART initiation: 0% of the time interval (group 1), 1%-99% of the time interval (group 2), and 100% of the time interval (group 3). The proportion of patients with undetectable HIV RNA, CD4+ cell count changes, and mortality were examined by logistic, linear, and Cox regression analyses, respectively. We constructed cumulative mortality curves. RESULTS: We observed 2046 patients, for a total of 8898 person-years of follow-up that started at 18 months after HAART initiation. Mean CD4+ cell count increase rates during 72 months of follow-up were as follows: group 1, 3.3 x 10(6) cells/L per month (95% confidence interval [CI], 2.9-3.7 x 10(6) cells/L); group 2, 2.9 x 10(6) (95% CI, 2.5-3.3 x 10(6) cells/L); and group 3, 2.6 x 10(6) (95% CI, 2.0-3.3 x 10(6) cells/L). Survival at 72 months were as follows: group 1, 92.7% (95% CI, 90.5%-94.4%); group 2, 85.6% (95% CI, 82.1%-88.5%); and group 3, 76.1% (95% CI, 70.6%-80.7%). At 72 months, 96% of group 1, 83% of group 2, and 57% of group 3 had an HIV RNA load of < 400 copies/mL (P < .01). Treatment interruption before baseline was a predictor of mortality in group 2 (adjusted rate ratio, 2.94; 95% CI, 1.75-4.92]). CONCLUSIONS: Viral suppression during the first 6-18 months after HAART initiation predicts viral suppression, CD4+ cell count progression, and survival at 72 months.