The general practice medical records of 214 children born in 1977 were scrutinised for a diagnosis of asthma. In 18 (8%) of these a diagnosis of asthma had been entered. Using a scoring system based on the medical record a further group of children who were thought likely to have undiagnosed asthma was exercise tested. Twelve children (6%) had demonstrable exercise induced asthma. In addition, seven children (3%) had both frequent respiratory symptoms and borderline exercise test results, indicating that they too had clinically important airways obstruction. As expected, histories of atopic eczema, nocturnal cough, persistent cough (more than one week), and wheezing appeared often in the medical records of the children with asthma. In combinations these diagnostic clues were more than 50% predictive of asthma. A more active approach in general practice to the diagnosis of asthma in children is both necessary and possible.