Theophylline has been demonstrated to be a useful agent in the therapy of chronic asthma. Its use must be tempered with knowledge of its adverse effects and that these effects are related primarily to serum concentration. Accordingly, it is mandatory to monitor serum theophylline concentrations on a regular basis with any patient receiving maintenance therapy with theophylline. It is also necessary to recognise the potential side effects of theophylline therapy, and when such a patient displays symptoms of vomiting, headache or seizures, serum theophylline concentration must be checked even if a recent concentration was within the therapeutic range. The means for monitoring theophylline concentrations are now available even to the average physician who does not have immediate access to a laboratory that can provide timely serum theophylline determinations.