Recent prevalence data for childhood asthma in Switzerland suggest a substantial underdiagnosis which seems to be more pronounced in girls. We further analysed our data trying to specify risk factors for underdiagnosis and undertreatment. Our special interest was focused on female sex as there is evidence for a sex-dependent diagnosis and treatment of chronic disease in adults, called the Yentl syndrome. The data are derived from a parent completed questionnaire survey of a stratified cluster sample of schoolchildren aged 7, 12, and 15 years. Besides the 12 months prevalence of asthma symptoms and bronchodilator treatment, the lifetime prevalence of an asthma diagnosis was noted. With a response rate of 97%, a total of 4353 completed questionnaires were analysed. While age was not associated with undertreatment (except for exercise-induced symptoms in adolescents), the lack of a formal diagnosis of asthma and atypical asthma symptoms other than wheeze such as chronic night cough were confirmed as significant risk factors for undertreatment. Of all boys reporting asthma symptoms 31% received bronchodilator treatment compared with only 15% of the symptom-reporting girls (P < 0.001). For all particular asthma-related symptoms (except wheeze), significantly more boys than girls (approximately double) received treatment. The physiological and psychological bases for these findings are discussed and suggest that gender is an important risk factor for underdiagnosis and undertreatment of asthma. Our research indicates that the Yentl syndrome may exist for childhood asthma.