The factors contributing to unequal mortality rates following Pneumocystis carinii pneumonia (PCP) in different groups at risk are poorly understood. We therefore compared the first episodes of PCP without prophylaxis in human immunodeficiency virus infected (HIV) and otherwise immunosuppressed patients in this retrospective study. A total of 58 HIV-infected and 16 otherwise immunosuppressed patients were analysed. The comparison included epidemiological, clinical, laboratory, radiological and microbiological data, as well as therapy and clinical course. A prognostic analysis was performed using a logistic regression model. The mortality was significantly different in the two groups (HIV group 17 versus non-HIV group 50%). Renal transplant patients had a higher survival rate as compared to malignancy or collagen vascular disease as underlying diseases at risk. Acute respiratory failure was more common in the non-HIV group. Variables found to be significantly associated with lethal outcome in univariate analysis were alveolar to arterial pressures difference for oxygen (P(A-a),O2), haemoglobin, platelet count, total protein, serum albumin, and gamma-globulins in the HIV-group, and serum albumin in the non-HIV group. In the multivariate analysis of the HIV group, platelet count and gamma-globulins remained independent prognostic factors. In conclusion, in the HIV-group, mortality is closely related to the severeness of PCP as well as to the severeness of the acquired immune deficiency syndrome (AIDS) disease. In the non-HIV group, malignancy and collagen vascular disease as underlying conditions at risk account for the high mortality rate. Its severeness was mainly reflected by serum albumin, which represented the only variable found to be significantly associated with death in both groups.