LDH levels were measured in 30 AIDS patients with P. carinii pneumonia (PCP), evidenced by bronchoalveolar lavage, and 12 HIV 1-infected patients with P. carinii-negative bronchial or pulmonary manifestations, constituting the control group. Extrapulmonary causes of elevated LDH levels were eliminated. In the case of bronchopneumopathy, the sensitivity and the specificity of an abnormal LDH level for suggesting PCP were both 83%. For an interstitial pneumopathy, the sensitivity was the same but the specificity was 100%. During a one year period, the prevalence of PCP in our department was 69%. The positive and negative predictive values of increased LDH levels in HIV-infected patients were, respectively: 92 and 63% for bronchopneumopathy, and 100 and 73% for interstitial pneumopathy. Furthermore, the lowering and then the normalization of the LDH value were observed in all PCP cases with a favorable outcome. This simple yet highly sensitive laboratory analysis should be used for the diagnosis and monitoring of all bronchopneumopathies in HIV-infected patients.