Evaluation of vigabatrin as an add-on drug in the management of severe epilepsy. The effects of the addition of Vigabatrin, a new anti-epileptic drug, to the therapy of 128 patients with severe medically refractory epilepsy is reported. Forty two (33%) of patients experienced side effects, which were predominantly neurotropic. In 28 (22%), the drug was withdrawn because of these side effects. The commonest side effects were drowsiness and behavioural change. The remaining 100 patients were followed for a mean of 30 weeks (range 12-75). Forty one of these patients showed a marked improvement in seizure frequency (a 50% or more reduction when compared with the pre-trial period), and nine (7%) were rendered seizure free. Apparent tolerance to the effects of the drug were noted in five patients. An exacerbation of seizures may occur if the drug is withdrawn too quickly. Vigabatrin appears to be a promising new anti-epileptic drug. 