There is controversy over the role of age of asthma onset in childhood asthma. Data collected on self-reported physician-diagnosed asthmatic children and young adults aged 6-24 years (N = 352), who participated in the second National Health and Nutritional Examination, 1976-80 (NHANES II), a national sample, were examined to see whether reported age at onset was associated with the future course of the asthma. Three definitions were used for early-onset asthma: asthma beginning before the second birthday, before the third birthday, and before the fourth birthday. Late-onset asthma was defined as asthma beginning on or after the second birthday, the third birthday, and the fourth birthday, respectively. Among 6-14 year olds, late-onset asthmatic subjects as compared with early-onset asthmatic subjects using the three definitions reported more allergic rhinitis OR = 3.79 (95% CI 1.53, 9.41), 3.06 (1.33, 7.07), 2.71 (1.18, 6.22), and were more likely to have at least one positive allergen skin test OR = 2.21 (95% CI 1.02, 4.79), 2.90 (1.29, 6.49), 3.41 (1.50, 7.75). Late-onset asthmatic subjects tended to report that their asthma was active, have more problems during the past 12 months with wheezing, and have lower values for predicted FVC and FEV1. No difference was found in reported chronic rhinitis, sinusitis, other allergies, problems within the last 12 months with cough attacks, or during the past 3 years a period of cough and phlegm lasting more than 3 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)