Serum lactic dehydrogenase (LDH) activity was compared with mortality in patients with the acquired immunodeficiency syndrome (AIDS) and Pneumocystis carinii pneumonia during the first four days of admission to assess the test's predictive value. In 30 admissions, 29 patients who survived an episode of Pneumocystis pneumonia had a mean LDH value of 385 IU, with five values greater than 520 IU. Eight with pneumonia who died had a mean value of 926 IU: all had values higher than 520 IU. The mean LDH values for 20 patients with AIDS (35 admissions) who survived and 4 who died of non-Pneumocystis disease were 240 IU and 350 IU, respectively; these patients were the control population. The positive and negative predictive values for survival using 520 IU as the threshold are 61% and 100%. Thus, LDH measurements in the first days of admission for P carinii pneumonia predict mortality and are useful in guiding future management.