To study the biochemical, familial, and clinical features of the fish odour syndrome among subjects with suspected body malodour. Subjects who responded to a newspaper article were screened for the fish odour syndrome by interview and biochemical tests. Families of subjects with the syndrome were tested if possible. St Mary's Hospital, London, and some interviews at subjects' homes. 187 subjects (28 males) with suspected body malodour, of whom 156 (19 males) underwent biochemical tests. Five families of six of the subjects with the fish odour syndrome agreed to further tests. Amounts of trimethylamine and trimethylamine N-oxide in urine collected over 24 hours under normal dietary conditions and for eight hours after oral challenge with 600 mg trimethylamine. The fish odour syndrome was diagnosed in 11 subjects: the percentage of total trimethylamine excreted in their urine samples that was oxidised to trimethylamine N-oxide was < 55% under normal dietary conditions and < 25% after oral challenge with trimethylamine (in normal subjects > 80% of trimethylamine was N-oxidised). Parents of six of the subjects with the syndrome were tested: all showed impaired N-oxidation of excreted trimethylamine (< 80%) after oral challenge, indicating that they were heterozygous carriers of the allele for the syndrome. The syndrome was associated with various psychosocial reactions including clinical depression. The fish odour syndrome can be inherited in an autosomal recessive fashion. It should be considered as a possible causative factor in patients complaining of body malodour.