A simple and rapid high-performance liquid chromatographic method has been developed for the determination of vigabatrin concentrations in plasma or serum. The assay uses only 100 microL of specimen and has been found to be linear over a concentration range of 1 to 50 mg/L. The limit of detection has been determined as 1 mg/L, and the between-batch coefficient of variation for the two internal quality controls routinely analysed (n = 33) has been found to be less than 5%. There was no evidence of interferences in the assay from other commonly prescribed anti-epileptic drugs. This method has been applied to routine clinical specimens to determine the concentration of vigabatrin in 47 patient specimens over a 12-month period. It was found that only 63% of the male group and 53% of the female group were within the proposed target concentration of 5 to 35 mg/L. In addition, it was found that 26% of the male group and 36% of the female group were found to have concentrations below 5 mg/L, which may indicate lack of compliance and/or lack of therapeutic efficacy of treatment.