1. Whilst the majority of individuals within a British white population are able to convert > 90% of their dietary-derived trimethylamine to its N-oxide, outliers exist who show varying degrees of decreased metabolism. Such individuals, excrete unoxidized trimethylamine in their urine and, if N-oxidation is sufficiently low, may experience malodour problems (Fish-Odour syndrome). 2. Such observations have now been extended to a much larger group (n = 421; 221 males) of British white volunteers recruited from staff and students of Imperial College Medical School at St. Mary's, London. Each subject collected a 0-24-h urine sample, which was subsequently analysed for total trimethylamine and trimethylamine N-oxide content. 3. Sixteen subjects (3.8% population; seven male, nine female) excreted < 90% of their total trimethylamine output as N-oxide. All six subjects who excreted < 80% as N-oxide (indicative of potential heterozygous status for deficient N-oxidation-fish odour syndrome) were female.