The effects of age and co-medication on steady-state carbamazepine (CBZ) level-dose (L/D) ratios were evaluated retrospectively in 370 paediatric patients. The CBZ L/D ratio increased significantly with age up to 15 years in patients on monotherapy (L/D = 0.051 x AGE + 0.334, n = 111, r = 0.593, P less than 0.001). Associated anti-epileptic therapy affected the CBZ L/D ratio, which was significantly reduced in patients on polytherapy as compared to patients on monotherapy. The study therefore suggests that routine monitoring of CBZ serum levels is extremely useful, especially in the paediatric age group, and in patients who require associated anti-epileptic medication.