1 Consecutive medical inpatients expected to benefit from a theophyllinate were treated with sustained-release aminophylline in a protocol conforming with ordinary practice. Of 16 patients, five had toxicity with aminophylline 450 mg daily, and a further three with 900 mg daily. Toxicity was serious in three patients. 2 Toxicity was significantly less common in cigarette smokers, and was related to higher plasma theophylline concentrations. However, there was a large overlap between concentrations associated with toxicity (as low as 9 micrograms/ml) and the accepted therapeutic range (5-20 micrograms/ml). Most patients with toxicity had theophylline levels within the therapeutic range. 3 For the same dose of aminophylline there was sevenfold variation between patients in plasma theophylline, with higher concentrations in non-smokers, infrequent alcohol users, older patients, those with left ventricular failure and those with lower serum transaminases. There variables could not be separated completely because of the small number of observations. 4 A nomogram for aminophylline dosage or monitoring of serum theophylline levels would have prevented little of the toxicity observed in these patients, although these measures would ensure that therapeutic concentrations were attained, and might prevent life-threatening toxicity.