The objectives of this study were to investigate underdiagnosis of asthma in adolescents and young adults in general practice and to examine the influence of patient characteristics. The results of screening on asthma (questionnaire, spirometry) were compared with the diagnosis made in general practice. Screening results served as the reference standard. Four general practices forming the Continuous Morbidity Registration, Nijmegen were used. Five-hundred and fifty-one subjects aged 10-23, registered since their birth till the year of the study (1989) on the practice lists of these practices were involved in the study. The data collected included: (i) all episodes of respiratory morbidity in their life presented in general practice; (ii) socioeconomic level and family history of asthma; and (iii) current respiratory status, allergy to inhalant allergens and smoking behaviour (questionnaire, spirometry, histamine-challenge test and Phadiatop-test). The screening identified asthma in 19% of the study subjects, of whom 56% not had been recognized in general practice. Of the asthmatics recognized in general practice, significantly more were boys, had presented acute bronchitis in the last 5 years preceding this study and had a family history of asthma than those not recognized. Of the latter, 30% had not presented any respiratory disease in the last 5 years to their general practitioner (GP). This study suggests underdiagnosis of asthma in adolescents and young adults. Asthmatics are more likely to be recognized by their GPs in case of male sex, previous diagnosis of acute bronchitis and a family history of asthma. Reluctance of some patients to consult their doctor and low perception of symptoms may play a role in underdiagnosis.