In 1956 a programme was initiated to vaccinate all children aged 12-14 years who were attending schools in Queensland, Australia. In view of the declining incidence of tuberculosis in Australia as a whole, there was a need to evaluate the effectiveness of the programme and its procedures. We therefore carried out a case-control study of Queensland's population, excluding certain known high-risk groups. Cases were Queensland residents with notified tuberculosis and of the appropriate age; two controls per case were chosen from the electoral roll. Information on vaccination status was obtained mainly from questionnaires and school records, where available. The results show that at best BCG vaccination had a modest protective effect, approximately 30% when the patients were diagnosed, which was on average 15 years after they had been vaccinated in the school programme. In the north the climate of Queensland is tropical, while in the more heavily populated south it is subtropical. A substantial proportion of the school records reported weak positive reactions to preliminary tuberculin testing, believed to be due largely to atypical mycobacteria. A similar phenomenon has been observed in other tropical regions, and may help to explain the apparent absence of a strongly protective effect for BCG vaccination.