We describe the case of an otherwise healthy 7-year-old girl whose mother noticed that she intermittently smelt of fish. This was due to the intermittent excretion of trimethylamine which could be precipitated by choline ingestion and by eating fish. Excluding eggs, liver and salt-water fish from the diet relieved the symptom. After a standard 15 g choline load, the child's father, but not her mother, excreted amounts of trimethylamine which were intermediate between those excreted by the patient and normal control subjects.