Warnings have been issued regarding safety in the use of theophylline preparations in patients receiving influenza vaccination. These were prompted by reports that influenza vaccination caused inhibition of metabolism of theophylline and aminopyrine. Previous work has indicated a role for ascorbic acid in mixed function oxidase activity, and ascorbic acid is commonly used by the public. We studied the effect of influenza vaccination and ascorbic acid supplementation on the rate of elimination of theophylline. In 11 patients and 12 normal subjects no significant effects of influenza vaccination or ascorbic acid supplementation on theophylline clearance, serum half-life, or apparent volume of distribution were detected (p > 0.3). We conclude that neither influenza vaccination nor ascorbic acid supplementation induce a predictable effect on theophylline disposition.