In 328 immunocompromised patients, 105 with and 193 without Pneumocystis jiroveci pneumonia (PCP), serum lactate dehydrogenase (LDH) was analysed retrospectively, taking into consideration the time interval from the onset of symptoms to the start of specific therapy. 97 of the 105 PCP patients were negative for human immunodeficiency virus (HIV). Eight were positive. Of the 193 patients without PCP 134 were HIV-negative and 59 were HIV-positive. In HIV-negative patients the sensitivity of LDH elevation was 63% and specificity 43%. In HIV-positive patients sensitivity was 100% and specificity 47%. The overall accuracy of LDH for the diagnosis of PCP was 52%, 51% in HIV-negative and 58% in HIV-positive patients. Except for its sensitivity in HIV-positive patients, the value of LDH for the diagnosis of PCP should not be overestimated.