A 63-year-old man was admitted to the hospital with fever and bilateral, peripheral infiltrates. Infectious disease and malignancy seemed to be excluded by fiberoptic diagnostic procedures. Subsequently, respiratory insufficiency developed, making open lung biopsy impossible. The diagnosis of bronchiolitis obliterans organizing pneumonia (BOOP) was strongly considered and treatment with corticosteroids was started; this led to dramatic clinical and radiologic improvement for a short time. Eventually, an open lung biopsy specimen disclosed primary extranodal non-Hodgkin's lymphoma of T-cell origin, immunoblastic, of high-grade malignancy according to the Kiel classification. After the first course of chemotherapy, total respiratory insufficiency developed and the patient died. This case is unique in a patient without AIDS.