A 52-year-old man presented with BOOP-like recurrent bilateral pulmonary infiltrates. In 1989 a chest X-ray film showed an infiltrative shadow in the right S6 region which disappeared after administration of prednisolone. Thereafter, bilateral patchy infiltrates recurred many times, and each time they resolved rapidly with steroid therapy. Skin eruptions on the face recurred. In October 1994 the patient underwent an open-lung biopsy of the infiltrate in the left S6 region. The pathological findings were consistent with BOOP, except for the moderate-to marked infiltration of lymphocytes. A lymphoproliferative disorder was suspected, and Southern blot analysis of the specimen revealed a rearrangement of the TCR-beta gene, which led to the diagnosis of T cell lymphoma. Ten months after the diagnosis, no recurrence of the lymphoma had been detected. In this case a gene analysis of the biopsy specimen was very useful for the diagnosis of T cell lymphoma.