Theophylline levels in plasma and saliva were compared in adults and children receiving oral theophylline preparations. There was a significant correlation (r = 0.91 for adults, r = 0.94 for children) between plasma and saliva theophylline concentrations, but when saliva levels were used to predict plasma levels, results were variable. Only 25% of adult samples and 53% of those from children lay within 91–110% of the actual measured plasma theophylline levels. Salivary theophylline concentration was found to be independent of salivary flow rate, although the effect of salivary composition on theophylline levels was not examined. Binding of theophylline to plasma proteins was constant at 43.4% over a concentration range of 1–25 μg/ml. The present findings suggest that although the use of saliva theophylline estimations is unlikely to prove a valid substitute for obtaining plasma levels in pharmacokinetic studies, salivary theophylline concentrations may be of value as a rough guide in routine clinic monitoring, particularly for assessment of compliance with therapy.