As the interpretation of tuberculin skin tests is controversial in subjects who have received BCG vaccine, we administered Mantoux tests to 2588 randomly selected Saudi children aged 5-13, 1945 of whom had been vaccinated with BCG at birth and 643 were unvaccinated. Only 7.8% of the BCG-vaccinated children were Mantoux positive (greater than or equal to 10 mm induration) at the age of 5 years, which was not significantly different from the unvaccinated children. The tuberculin sensitivity rose more steeply with age in the BCG-vaccinated than the unvaccinated children so that the difference between both groups became statistically significant in those aged 12 and 13 (20% versus 3.9%, 15.5% versus 4.1% respectively). These findings support the previously expressed theoretical postulates that BCG-vaccinated subjects display an increased ability to respond immunologically to encounters with environmental mycobacteria. In communities with low prevalence of environmental mycobacteria, this would result in a slow but persistent rise of skin reactivity to tuberculin which, if given time, will become greater than that of unvaccinated subjects.