1. Beginning with a single propositus, who had been previously diagnosed at the age of 10 as suffering from trimethylaminuria (fish-odour syndrome), both her parents and two sisters were investigated biochemically with respect to their ability to N -oxidize trimethylamine (TMA), both when derived from the diet and when administered exogenously. 2. Both the propositus and a second sister were markedly deficient in their ability to N -oxidize TMA, both when derived from the diet and when given as such; furthermore, both siblings readily developed the symptoms of fish-odour syndrome as characterized by a strong objectionable breath and body odour shortly after the oral administration of TMA (300 mg). 3. At this dose level of TMA, neither of the parents nor the third sister showed any evidence of impaired N -oxidation ability nor did they experience any ‘fish-odour’ symptoms. 4. With an oral challenge of 600 mg of TMA, both the parents showed a clear impairment of N -oxidation capacity which was not seen in six healthy unrelated volunteers. Both parents experienced a fish-odour syndrome at this level of TMA challenge. 5. The family data support the hypothesis that trimethylaminuria is an inborn error in the ability to N -oxidize TMA which is inherited as an autosomal recessive trait. Furthermore, experience with this family suggests that an oral challenge dose with 600 mg of TMA may be used to identify carriers of the condition.