OBJECTIVES: To investigate the efficacy, tolerability, and kinetics of lamotrigine during the first year of life. STUDY DESIGN: We studied 13 infants with intractable seizures; 7 had partial seizures and 7 had infantile spasms (1 had both). Patients received open-label lamotrigine as add-on therapy for 3 months. Seizure frequency, response ratio, and side effects score were determined and compared with the baseline period. RESULTS: The rate of partial seizures per day decreased from 8.57 +/- 2.29 to 4.00 +/- 2.15 (P =.027) and infantile spasms from 8.71 +/- 2.15 to 3.61 +/- 2.762 (P =.028). Apparent clearance increased during the first year of life, with a break point at 2 months of age (mean, 0.119 +/- 0.021, 0.217 +/- 0.094 L/h per kilogram for infants <2 months and those 2 to 12 months old, respectively,P <.001). Twenty-four-hour concentration to time plots of three 3- to 4-week-old neonates showed a half-life of 23.44 +/- 3.57 hours. Compared with a group of 17 older children, LTG had similar efficacy (response ratios, -0.68 +/- 0.12 and -0.74 +/- 0.11, P =.504), and similar adverse effects scores (0.67 +/- 0.67 and 0.23 +/- 0.166, P =.95). CONCLUSIONS: Lamotrigine is a useful and well tolerated drug for partial seizures and infantile spasms in infants <1 year of age. However, lamotrigine has age-dependent kinetics that must be taken into consideration.