Introduction: New challenges have arisen for the management of critically ill HIV/AIDS patients. Severe sepsis has emerged as a common cause of intensive care unit (ICU) admission for those living with HIV/AIDS. Contrastingly, HIV/AIDS patients have been systematically excluded from sepsis studies, limiting the understanding of the impact of sepsis in this population. We prospectively followed up critically ill HIV/AIDS patients to evaluate the main risk factors for hospital mortality and the impact of severe sepsis on the short- and long-term survival. Methods: All consecutive HIV-infected patients admitted to the ICU of an infectious diseases research center, from June 2006 to May 2008, were included. Severity of illness, time since AIDS diagnosis, CD4 cell count, antiretroviral treatment, incidence of severe sepsis, and organ dysfunctions were registered. The 28-day, hospital, and 6-month outcomes were obtained for all patients. Cox proportional hazards regression analysis measured the effect of potential factors on 28-day and 6-month mortality. Results: During the 2-year study period, 88 HIV/AIDS critically ill patients were admitted to the ICU. Seventy percent of patients had opportunist infections, median CD4 count was 75 cells/mm3, and 45% were receiving antiretroviral therapy. Location on a ward before ICU admission, cardiovascular and respiratory dysfunctions on the first day after admission, and the presence of severe sepsis/septic shock were associated with reduced 28-day and 6-month survival on a univariate analysis. After a multivariate analysis, severe sepsis determined the highest hazard ratio (HR) for 28-day (adjusted HR, 3.13; 95% CI, 1.21-8.07) and 6-month (adjusted HR, 3.35; 95% CI, 1.42-7.86) mortality. Severe sepsis occurred in 44 (50%) patients, mainly because of lower respiratory tract infections. The survival of septic and nonseptic patients was significantly different at 28-day and 6-month follow-up times (log-rank and Peto test, P < 0.001). Conclusions: Severe sepsis has emerged as a major cause of admission and mortality for hospitalized HIV/AIDS patients, significantly affecting short- and longer-term survival of critically ill HIV/AIDS patients.