A total of 740 healthy children aged between 3 months and 11 years who had received BCG vaccination in the first month of life were Mantoux tested in Sri Lanka. Despite 97% having a visible scar following vaccination, 80% showed Tuberculin anergy (0-1 mm). Those without a scar showed no response. There was no correlation between scar size and the Mantoux response. A low mean Mantoux reaction was seen at all ages: 3.5 mm at 3 months, 3.2 mm at 18 months, 1.8 mm at 5-7 years and 1.9 mm at 9-11 years. A significant waning of the Mantoux reaction occurred at 5-7 years but there was no significant change at 9-11 years. Children who had received routine revaccination at 10 years, having received the first at birth, had a significantly higher mean Mantoux reaction (9.6 mm) when tested 3 months after revaccination. In 90 bacteriologically proven tuberculous patients, there was a significantly increased Mantoux reaction compared to all other groups. 1 T.U. PPD RT 23 (with tween 80) was used in all instances for Mantoux testing. The present study shows that routine BCG vaccination at birth, using a reduced dose of 0.05 ml, is unlikely to interfere subsequently with the diagnostic value of the Mantoux test. However, revaccination at 10 years may do so.