Serum lactate dehydrogenase levels, alveolar-arterial oxygen gradient, and percentage of neutrophils in bronchoalveolar lavage correlate most strongly with early mortality in Pneumocystis carinii pneumonia (PCP) in HIV-infected patients. However, the individual outcome can not be predicted by these parameters due to a considerable overlap between survivors and nonsurvivors. We prospectively investigated a PCP severity score, which has been developed earlier based on a retrospective analysis. Seven of 94 consecutively examined HIV-infected patients died within 14 days after diagnosis of PCP. A PCP severity score greater than 7 had a positive predictive value for early fatal outcome of 66.7 percent (6/9) and a negative predictive value of 98.8 percent (84/85). The overall diagnostic accuracy was 95.7 percent (90/94). The positive predictive value for early fatal outcome of a P(A-a)O2 > 35 mm Hg was 24 percent (6/25); the negative predictive value was 98.6 percent (68/69). However, the overall diagnostic accuracy was only 78.7 percent (74/94). The PCP severity score is a valuable tool for clinical decision making, for the early identification of patients with a prognostic unfavorable course, and for the comparison of patient populations in future studies of HIV-associated PCP.