The paper presents a detailed analysis of errors in the diagnosis of bacteriologically negative pulmonary tuberculosis in a random sample of 560 patients out of all such patients registered in 1993. The false diagnosis was found in 63 patients i.e. in 11.3% of the sample. Among the 63 patients with false diagnosis of tuberculosis there were 15 cases with lung cancer, 15 cases with pneumonia and/or pleuritis, 15 cases with disease of circulatory system with abnormal radiological of the lungs, 8 cases with old post-tuberculosis changes in the lung considered as relapses, 4 cases of bronchiolitis obliterans with organizing pneumonia, 3 cases of sarcoidosis and 1 case of aspergillosis in a post-tb cavity. The most serious errors were those related to failure in diagnosing (or of too late diagnosis) of lung cancer. These failures comprised 2.7% of patients in the sample. The chance to detect a lung cancer in the sample was 2900 cases in 100,000 men and 2200 cases in 100,000 women. This chance was much higher than in general population--29x for men and 100x for women. The main source of errors was false interpretation of radiological examinations and neglect to utilise other diagnostic procedures like bronchoscopy, cytology or tomography examinations. As an outcome of the analysis authors present their recommendations for diagnostic procedures in the diagnosis of bacteriologically negative pulmonary tuberculosis.