Trimethylamine (TMA) and its N-oxide (TMAO) are normal components of human urine. They are present in the diet and also derived from the enterobacterial metabolism of precursors such as choline. Dietary TMA is almost entirely metabolized to and excreted as TMAO. However, the extent to which TMA undergoes N-oxidation appears to be polymorphic in a British white population study (n = 169). Two propositi were identified with relative TMA N-oxidation deficiency that was further confirmed by oral challenge with TMA (600 mg). The study of the families of the two propositi, as well as those of two identified subjects with trimethylaminuria, under both normal dietary conditions and after oral TMA challenge strongly indicates that the conditions of impaired N-oxidation is inherited as a recessive trait. It is proposed that the N-oxidation of TMA in humans is polymorphic and under single gene diallelic control in which individuals who are homozygous for the variant allele exhibit marked N-oxidation deficiency and trimethylaminuria.