OBJECTIVE: To evaluate the ability of a variety of scoring systems to predict mortality of patients admitted to an intensive care unit (ICU) with acute respiratory failure (ARF) secondary to AIDS-related Pneumocystis carinii pneumonia (PCP). METHODS: All patients with AIDS-related PCP admitted to ICU at St. Paul's Hospital between January 1, 1985 and April 1, 1991 were reviewed. For each case, the following scores were calculated from data obtained within 24 h of ICU admission: acute physiology and chronic health evaluation II (APACHE II); acute lung injury score; AIDS score as described by Justice and Feinstein; and modified multisystem organ failure (MSOF) score. The serum lactate dehydrogenase (LDH) level was also recorded when obtained within 24 h of ICU admission. RESULTS: A total of 52 ICU admissions in 51 patients were studied. Overall mortality was 65 percent. Mortality increased with increasing MSOF (p < 0.05) score and LDH (p < 0.05). Based on receiver operating characteristic (ROC) curves, the MSOF score and the LDH were found to be good predictors of mortality. Multivariate logistic regression showed that the MSOF score was the only independent predictor of mortality (p < 0.05). The AIDS score, APACHE II, and the acute lung injury score were not significantly associated with mortality. Addition of the serum LDH level improved the performance of both the MSOF and AIDS scores, though the AIDS score plus LDH performed no better than the LDH alone. Of all the scores tested, the MSOF plus LDH level was the best (p < 0.005) predictor of mortality. CONCLUSIONS: The modified MSOF score and the serum LDH level are the best predictors of mortality of patients admitted to ICU with ARF secondary to AIDS-related PCP. The performance of the MSOF score was enhanced when the LDH level was added. The AIDS score, APACHE II, and the acute lung injury score were not found to be useful in this group of critically ill patients.