Boys suffer more often from asthma than girls, while in adults the gender ratio is reversed. It is not clear when exactly this change occurs and by what mechanism. From a cohort of all 5030 German 4th grade pupils (age 10 years) in Munich, 274 children with current asthma were identified (164 males, 110 females) through a questionnaire, and skin prick tests were performed. These subjects were re-evaluated at ages 14 and 20 years with a questionnaire. A random sample (n = 1000) of all 3538 German children without current asthma at age 10 was also re-evaluated at age 20 (controls). At age 20, only 24.5% (21 males, eight females) of the initial asthma group still had symptoms, and their gender ratio remained male dominated. In the controls, 4.8% (48/1000) had current asthma at age 20 and these were predominantly female: 6.4% (31/485) of control girls vs. 3.3% (17/515) of boys (p = 0.022). Half of the new asthma cases had had no symptoms or diagnoses until age 10, and atopy at age 10 was not associated with subsequent asthma in these. Asthma at age 10 has no better prognosis in boys than in girls, and the mechanism of the changing gender ratio appears to be late incidence of asthma among girls. Because the latter constitute a considerable part of adult asthma cases, it appears important to further explore this asthma phenotype and the risk factors associated with it.