Although bronchial asthma is one of the most common chronic illnesses of children and young adults, it remains underdiagnosed. To assess the value of the chest X-ray in helping to make the diagnosis of asthma at the primary care level, we studied the medical records of 58 patients aged 18-40 with the diagnosis of mild to moderate asthma and for whom both simple spirometry-forced one-second expiratory volume (FEV1) and/or peak expiratory flow rate (PEFR)--and a chest X-ray had been performed. Only 21 of 58 (36%) had spirometry indicative of asthma (SPI+) while 34 of 58 (59%) had abnormal chest X-rays (CXR+)--"increased markings" and/or "low diaphragm." Although CXR+ discriminated between asthmatic patients and a normal control group, no difference was found between asthmatic patients and a group of patients with acute bronchitis. Nonetheless, the number of asthmatic patients with CXR+ and SPI - (n = 23) was significantly larger than the number with CXR- and SPI+ (n = 10), which indicates that for mild asthma the chest X-ray may be more sensitive than spirometry even though not as specific. These results were surprising at the time of the investigation. Subsequently, however, the importance of the inflammatory response in asthma came to light, which rendered the results more interesting than surprising. We conclude that the chest X-ray has value in making the diagnosis of mild bronchial asthma.