The suppression of cellular immune responses during measles is thought to contribute to the development of secondary infections which often complicate this disease. To determine whether there was a difference in the altered cellular immune responses of children with and without complications we performed a prospective study of purified protein derivative skin test reactivity in children with natural measles virus infections who had received Bacillus Calmette-Guerin as infants. Twenty-five tuberculin-positive children who developed measles (13 uncomplicated and 12 complicated) were skin-tested weekly beginning 1 to 2 weeks before and ending 2 to 7 weeks after the onset of the rash. All children became anergic during the acute phase of measles. Children with complications remained unreactive for a significantly longer period of time after the rash (mean, 4 weeks) than did children without complications (mean, 2.3 weeks, P less than 0.001).