Children with asthma frequently have exercise-induced disease that can limit their participation in both organized sports and vigorous free play. We measured fitness in a group of children with moderately severe to severe asthma with an instrument that is used widely and is available to clinicians for assessment of the physical capability of their patients. Abnormalities in physical fitness were present primarily in the area of endurance, with performance in the nine-minute run frequently found to be low. Increased skin-fold thickness was also present more frequently than in the normal population. Abdominal strength and flexibility were normal. Abnormalities in fitness were not well explained by measures indicating the presence of poorly controlled disease. All children with significant asthma probably should be tested to document the level of fitness so that appropriate intervention can be initiated.