A 41-year-old man presented with history of a fishy odor that had been commented on by friends since he was 13 years old. Neither the patient nor his family members has been able to detect this malodor, which was a cause of considerable social embarrassment to the patient. He had used topical antiperspirants without much benefit. His medical history revealed a minor head injury, arthroscopy and meniscectomy, corneal foreign-body removal, reflux esophagitis, a duodenal ulcer, and panic attacks, the latter treated with propranolol hydrochloride. The family medical history revealed parental consanguinity, a brother who thought he had the same condition as our patient, and an unaffected sister. No malodor or skin abnormalities were detected at the initial examination but, because the clinical suspicion was that of trimethylaminuria (fish-odor syndrome), he was evaluated for that disorder. While on his normal diet, the urinary trimethylamine (TMA) level was 14.7 μmol/mmol of creatinine (reference range, 1.5-11.0 μmol/mmol). The TMA-N-oxide level was 34 μmol/mmol of creatinine (reference range, 17-147 μmol/mmol), and the TMA/TMA-oxidase ratio was 0.43 (reference range, 0.01-0.21). The significantly high excretion of TMA with a raised TMA/TMA-oxidase ratio was consistent with the diagnosis of primary trimethylaminuria. The patient was referred to a dietitian for advice on foods that contain low amounts of choline and lecithin. Two months later, his TMA level was 9.6 μmol/mmol of creatinine (reference range, 1.5-11.0 μmol/mmol), which reflected successful dietary modification. However, his urinary TMA level was elevated at 102.8 μmol/mmol of creatinine; the TMA-N-oxide level was 263.4 μmol/mmol of creatinine, and the TMA/TMA-N-oxide ratio was 0.39 when investigations were subsequently repeated, possibly a reflection of dietary relaxation. He received a trial dosage of 500 mg of oral metronidazole twice daily for 10 days, which did help to some degree in reducing the degree of odor. The most useful therapeutic approach, however, had been the fact that he was able to communicate with family members and friends about the condition, which he subsequently read about on the Internet. Molecular genetic studies confirmed homozygosity for a mutation in exon 4 of the flavin-containing monooxygenase 3 (FMO3) gene, FMO3/P153L (c.458C → T), a mutation known to be associated with complete loss of FMO3 enzyme activity.