A 16-year-old left-handed male is presented with a history of seizures associated with a fish-like odour and behavioural disturbances thought to be related to trimethylaminuria. His seizures were complex-partial (cursive) seizures and started at the age of 18 months. They occurred in the context of discrete episodes several times per year. The episodes would start with a fish-like odour, followed by seizures occurring in clusters and behavioural disturbance consisting of agitation, mixed affective symptoms, auditory hallucinations and delusions. A urinary assay of trimethylamine (TMA) was elevated, confirming the diagnosis of trimethylaminuria in this patient. He was treated with a choline-restricted diet with resolution of his symptoms. The occurrence of seizures and psychiatric disturbance in this patient was thought secondary to his trimethylaminuria due to the temporal relationship of his seizures and psychiatric disturbance with the odour and his response to treatment. The possible relationship of trimethylaminuria to seizures and to psychiatric disturbance are discussed and a review of the literature presented.