Objectives: To ascertain whether prolonged suppression of viral replication can be achieved in clinical practice and to identify factors associated with virological outcome. Design: Retrospective observational study. Setting: University-affiliated human immunodeficiency virus (HIV) clinic in Cleveland, Ohio. Participants: Patients treated with regimens that included protease inhibitors between June 1995 and December 1997. We identified 366 patients; 310 had sufficient virological follow-up data to be included. Main Outcome Measure: Virological success was defined as plasma HIV-RNA levels lower than 400 copies/mL at the last clinic visit. Virological failure was subdivided according to the maximum degree of suppression of vital replication achieved. Multivariate analysis was performed to identify baseline factors associated with virological outcome. Results: Virological success was achieved by 47% of patients at a median follow-up of 335 days. The median CD4+ cell count increase and HIV-RNA level decrease were 0.10 x 109/L (100 cells/μL) and greater than 1.3 log10 in patients who achieved virological success, and 0.010 x 109/L and 0.32 log10 for those who did not. In multivariate analysis the likelihood of virological success was diminished in women (P+ cell count (P+ cell count increase. Missed clinic visits, more advanced disease, and higher plasma HIV- RNA levels may predict failure.