Pulmonary Kaposi's sarcoma (KS) was diagnosed by pulmonary biopsy in a heterosexual parenteral drug abuser (PDA). The patient had previously been diagnosed of Pneumocystis carinii pneumonia and pulmonary tuberculosis. Thoracic computed tomography (CT) showed bilateral nodular lesions which were less apparent in conventional radiological study and which increased in size in spite of correct therapy. As cutaneous lesions suggesting KS subsequently appeared, the possibility of pulmonary KS was considered and confirmed by open biopsy. The rarity of a primarily pulmonary presentation of KS in a PDA, the difficulty in diagnosis owing to concomitant infective diseases and the diagnostic value of thoracic CT for the diagnosis of pulmonary KS are discussed.