Transcutaneous collection of theophylline and its metabolite, caffeine, was undertaken in 33 preterm infants (2 to 89 days old) who were receiving routine theophylline therapy. Collection was done by means of a novel adhesive transcutaneous collection system. The transcutaneous collection system accumulated substances that migrated from the blood to the skin surface by trapping them in an activated charcoal-gel matrix. On one to three occasions, four transdermal collection systems were applied to the back or abdomen of each infant for 4 to 12 hours. During that time, blood samples were obtained for routine monitoring of plasma theophylline levels. Amounts of theophylline (95 +/- 198 ng) and caffeine (83 +/- 77 ng) in the transcutaneous collection system were significantly correlated with the respective average plasma drug concentration and postconceptional age (p less than 0.01). Skin reactions were limited to mild erythema. We concluded that theophylline and caffeine can be collected on the surface of the skin of preterm infants with a novel transcutaneous collection system. Amounts collected by means of the transcutaneous collection system correlated with plasma concentrations consistent with a diffusion process, but they were poor predictors of individual concentrations.