A sample of 7465 persons aged 10 to 17 years from the 1988 National Health Interview Survey on Child Health was used to assess the prevalence and impact of chronic conditions in adolescents. We defined a condition as chronic if it was first noted more than 3 months before the interview or a condition that ordinarily would be of lengthy duration, such as arthritis or heart disease. An estimated 31.5% of US adolescents were reported to have one or more chronic conditions. The most commonly reported chronic conditions included respiratory allergies, asthma, and frequent or severe headaches. Chronic conditions had widely varying impact on adolescent activity levels. On average, adolescents with chronic conditions experienced 3.4 bed days and 4.4 school absence days related to their chronic conditions in the year before the interview. Adolescents with chronic conditions were also reported to experience 35% more behavioral problems than their counterparts without chronic conditions. Adolescents with multiple chronic conditions had substantially more bed days, school absence days, and behavioral problems than adolescents with a single chronic condition. Implications of these findings are discussed.