A 22-year-old woman was hospitalized because of fever of 39 degrees C and increasing dyspnoea. The chest radiograph demonstrated coarse confluent opacities bilaterally. Despite antibiotic treatment the condition deteriorated acutely after 2 days. All efforts to find an infectious agent, including immunological tests, were unsuccessful. Artificial ventilation became necessary because of increasing respiratory failure with an arterial oxygen partial pressure of 56 mm Hg, CO2 of 41 mm Hg and a respiratory rate of 60/min. Histological examination of a transthoracic lung biopsy revealed bronchiolitis obliterans organizing pneumonia, which was treated with prednisolone. The initial dose was 500 mg/d, gradually reduced to 12.5 mg/d over 2 weeks. The clinical and radiological findings improved markedly after 2 days and the patient discharged herself after 3 weeks and there was no follow-up.