Tuberculosis is still an important health problem in developing countries. A screening program was conducted upon learning that one of the teachers of a primary school in Ankara was diagnosed to have active pulmonary tuberculosis. A total of 341 students in the same building with the index case were screened for tuberculosis. There were 109 students with positive tuberculin test reaction. A higher ratio of tuberculin test positivity among the students of the teacher with active tuberculosis versus students vaccinated with BCG one year previously according to the routine vaccination program was determined. Isoniazid prophylaxis was given to the students with positive tuberculin test. The study shows the importance of an urgent work-up of index cases and their environment to prevent the spread of tuberculosis.