The level of serum lactate dehydrogenase (LDH) has been reported to be useful as a marker of Pneumocystis carinii pneumonia (PCP) in patients infected with the human immunodeficiency virus (HIV). In this study, we evaluated the clinical role of measurements of LDH in determining diagnosis and prognosis in 84 patients infected with HIV who presented with pulmonary problems. The mean serum LDH level of the 54 patients with PCP was 361 IU, which was significantly higher than the mean of 224 IU found in the 30 patients with other causes of pulmonary disease (p less than 0.001). Overlap of individual values occurred between the 2 groups. Levels greater than 450 IU consistently predicted PCP in our population, however, and normal values were found in only 7% of the patients with PCP. LDH levels in a group of 8 patients who developed PCP from conventional causes of immunosuppression were similar to those seen in PCP associated with HIV. LDH determinations prior to the episode of PCP were available in 45 patients and 39 (87%) showed an increase of greater than 50 IU at the time of diagnosis. The mean LDH in those who survived PCP was 340 IU, which was significantly less than the mean of 447 IU found in those who died (p less than 0.05). Significant overlap between the groups precluded use of the LDH as a predictor of mortality in an individual patient. Serial determinations of LDH during treatment for PCP showed that 27 of 36 (75%) of the survivors had gradual decreases of LDH, whereas 9 of 12 (75%) nonsurvivors had rising values during treatment.(ABSTRACT TRUNCATED AT 250 WORDS)