Vigabatrin is an anticonvulsant drug with a relatively favourable side-effect profile. However, in clinical trials behaviour disorders have been reported, including agitation, depression and psychoses. In this study, 136 cases of behavioural problems that had been reported to the manufacturers, or the authors, were followed up. Satisfactory clinical information could be obtained on 81 patients. Of these, 50 cases met the criteria for either a psychosis (n = 28) or depression (n = 22). These were compared with a group of Queen Square patients, with epilepsy and psychosis, who had never taken vigabatrin (n = 21) and another group, who received vigabatrin without experiencing any behavioural problems (n = 28). The main results from this study suggest that: (1) Psychosis as a treatment emergent effect of vigabatrin is seen in patients with more severe epilepsy, compared with those patients who never develop psychopathology, and those developing an affective disorder. The psychosis is related to a right-sided EEG focus, and suppression of seizures (64% became seizure free). ( 2) Depression as a treatment emergent effect of vigabatrin is associated with a past history of depressive illness. There was little or no change in seizure frequency in this group. Some suggestions for managing patients who may develop these behaviour disorders are given.