We studied 17 preterm infants receiving caffeine, and measured their plasma levels of caffeine and the theophylline metabolite by high-pressure liquid chromatography. The half-life was calculated by computer analysis using the least-square method. The mean gestational age of our patients was 29.7 ± 1.9 weeks (mean ± SD) and they were studied at 20.7 ± 6.6 days (mean ± SD) postnatal age. The caffeine half-life was 52.03 ± 23.87 h (means ± SD) and the theophylline half-life was 77.04 ± 65.01 h (mean ± SD).