The influence of liver volume and body weight on plasma theophylline clearance was investigated in asthmatic children and adults. A linear relationship (r = +0.99) was demonstrated between ideal body weight and liver volume estimated by an ultrasonic scanning technique. Age-related changes in liver volume-to-body weight ratio could account for only 30% of differences in plasma theophylline clearance (1/h/kg) between children and adults. It is summized that increased hepatic mixed function oxidase activity is the major contributor to the higher plasma theophylline clearance in children.