BASIC PROBLEM AND OBJECTIVE In rare instances Pneumocystis carinii pneumonia (PCP) can occur in a cystic form, characterized by an acute course. The correct diagnosis is often missed. An analysis of a large cohort of patients with PCP was undertaken to determine the incidence and clinical manifestations of this form of the disease. PATIENTS AND METHODS Data on all HIV-positive patients on the hospital registry, hospitalized with the diagnosis of PCP between January 1988 and July 1996, were retrospectively analysed. To be included in the study, PCP had to have been confirmed microscopically on bronchial lavage or histologically by lung biopsy. The chest radiogram and (if available) computed tomogram of these patients were examined for possible cystic changes, and clinical as well as laboratory findings recorded. RESULTS Five of 180 patients with PCP had cystic changes (2.8%). The interval between first symptoms and diagnosis ranged from 30 to 200 days (median: 32 days). All five patients (age: 24-49 years; all males) had a far advanced immunological disorder (median CD4 count: 10/microliter) and were in stage C3 (Centers for Disease Control). Three of the five patients had prophylactically inhaled pentamidine (300 mg/month). Bronchial lavage was false-negative in three of the five cases. One patient had a spontaneous pneumothorax. Lactate dehydrogenase levels were normal or only slightly elevated (207-417 U/l; median 305 U/l). Both the cavities and thin-walled cysts were demonstrated radiologically, but were more visible on CT than on the X-ray film. All patients responded well to the medication with cotrimoxazole, but the prognosis was poor with a survival time of only 0.3 to 16 months. CONCLUSION PCP should be included in the differential diagnosis when HIV patients have cystic lung changes. Invasive diagnostic methods are often required to avoid wrong diagnosis and superfluous therapeutic interventions.