We retrospectively evaluated the factors influencing the isolation of tubercle bacilli in 51 children under 14 years of age with pulmonary tuberculosis who were admitted to Yokohama City University Hospital from 1975 to 1998. Young children (0-6 years of age) with pulmonary tuberculosis were significantly less positive by smear and culture than elder children (7-14 years of age) with pulmonary tuberculosis. According to the Japanese Society for Tuberculosis classification of finding on chest X-ray film for pulmonary tuberculosis, the culture-positive patients with type II (cavitary lesions) were found in all, the culture-positive patients with type III (non-cavitary lesions) in 39.3%, and the culture-positive patients with type H (hilar and mediastinal lymphadenopathys) in 35.3%. Patients with cavitary lesions (type II) were significantly more positive by smear and culture than patients with non-cavitary lesions (type III + type H). Only 15.8% of the young children with pulmonary tuberculosis had received BCG vaccine and all had non-cavitary lesions (type III and type H). But, 84.6% out of the older children had received BCG vaccine and half had cavitary lesions. Taken together, the result was that there were few isolation of tubercle bacilli in young children with pulmonary tuberculosis because they had non-cavitary tuberculosis without delayed-type hypersensitivity to tubercle bacilli.