Forty patients with a history of haemoptysis, normal chest radiographs apart from evidence of chronic airflow limitation, and normal fibreoptic bronchoscopy (or blood alone in the bronchial tree) were investigated by computed tomography (CT). Abnormalities were seen in 20 (50%) of the CT scans. Seven of the patients had evidence of bronchiectasis (18%), one of whom also had a mass. In four (10%) cases a mass alone was detected (two tuberculous, two malignant). In a further four (10%) scans alveolar consolidation was present and in three cases abnormal vessels were detected (7.5%). One patient had cystic changes shown in their scan and multiple nodules were shown in the final patient. The contralateral lungs of 93 patients undergoing CT for pre-operative assessment of bronchogenic carcinoma were used as controls. In six (6%) of these patients abnormalities were detected by CT. Pleural nodules were observed in two patients, fat in the transverse fissure in another, atelectasis in two patients and an apical bulla in the other abnormal scan. The relative risk for patients with unexplained haemoptysis having abnormal CT scans compared to the control group of patients was 7.75. We conclude that computed tomography is of value in the investigation of patients with unexplained haemoptysis.