A total of 302 Chinese patients were diagnosed on clinical and radiographic grounds by chest physicians from the Hong Kong Chest Service as having radiographically active pulmonary tuberculosis, but had sputum negative for acid-fast bacilli on 5 recent microscopical examinations. They were not given antituberculosis chemotherapy until active disease had been confirmed by positive bacteriological findings, or by radiographic or clinical deterioration during close observation. Of the 283 patients assessed up to 30 months, 200 (71%) had active disease confirmed and had chemotherapy started during the 30 months. A further 42 (15%) had evidence of changing lesions on serial chest radiography, and hence of recently active disease. A number of characteristics of the patients and of their bacteriological and radiographic status were tested singly and in combination for association with the presence of active disease confirmed on admission or at any time during the 30 months. Patients with radiographic lesions which were larger and classified as "active" on independent radiological assessment, and with a history of blood-streaked sputum or frank haemoptysis were more likely to have unquestionably active disease on admission or at some time during the 30 months, than patients without these characteristics.