The influence of cigarette smoking on plasma theophylline elimination half-life was examined in a group of elderly in-patients, to determine whether the modifying effect of cigarette smoking is also present in this age-group. The plasma theophylline half-life was determined in five non-smokers and seven smokers aged 66-88 years after administration of 320 mg of theophylline syrup. A wide range of plasma theophylline half-lives was observed in each group. However, there was a significant difference (P < 0.05) between the mean plasma theophylline half-life in smokers (5.41 h, SEM 0.69 h) and non-smokers (10.40 h, SEM 1.90 h). It was concluded that, although plasma theophylline half-life tends to be longer in this age-group, the modifying influence of cigarette smoking on plasma theophylline half-life can be demonstrated in the elderly, as well as in young adults.