One hundred and twenty-seven children, aged 6 months - 14 years, attending a day-care centre in Pretoria had two Mantoux tuberculin tests performed 8 weeks apart. On initial testing 19.7% of the children had reactions greater than or equal to 10 mm and positive tests were commoner in the older children--who had received BCG vaccination twice. On repeat testing a significant increase in the mean tuberculin reaction size was noted and 13% of the children converted to tuberculin positivity. Since an extensive search revealed no evidence of active tuberculosis in either children or adults at the day-care centre, it was concluded that the observed enhancement of the tuberculin reactions was due to the booster effect. This phenomenon was most marked in preschoolers with a BCG scar. It is important to recognise that boosting can occur in children and may be confused with true conversion to tuberculin positivity caused by infection with Mycobacterium tuberculosis.