H2-antagonists such as cimetidine and ranitidine are metabolized by cytochrome P-450. In this way they may interfere with theophylline metabolism. Cimetidine is known to have this effect and frequently to induce a theophylline toxic effect, while data concerning ranitidine are more uncertain. In this paper, we report the case of a 67-year-old woman with non-insulin dependent diabetes. She was taking aminophylline for respiratory failure and after ranitidine infusion exhibited generalized convulsions. Theophylline values which were monitored within the therapeutic range, increased toxic levels after ranitidine therapy and epileptic episodes. The increase in theophylline levels was associated with a further reduction in the clearance rate of the bronchodilator. We think that ranitidine may combine with other clinical factors known to reduce theophylline metabolism mainly in the elderly and severely ill patients. Theophylline-induced seizures may occur when theophylline serum levels are slightly above the therapeutic range, as in our case report.