This study examined the natural history of reaction after accidental intradermal administration of bacille Calmette-Guerin (BCG) vaccine instead of purified protein derivative (PPD) in 226 schoolchildren. At 18 days after vaccination, a local reaction with a diameter of 4.5-14 mm was found in 62% of the students, and ulceration with discharge was found in 26.6%; corresponding rates at 120 days were 72.3% and 38% and at 281 days were 73% and 6%. At 345 days, 85% of the students had a dry scar measuring 5-14 mm in diameter, and none had ulceration or discharge.