In January, 2006, a 45-year-old man presented to our department; he complained of a strong objectionable body odour, resembling the aroma of fi sh, which had a profound infl uence on his life. He decided to seek help, because of marital problems related to his body odour. His wife noticed that the smell was present in his sweat, urine, semen, and saliva, and it got worse with consumption of certain food products. He had been off ered a better job, but declined when he noticed that he had to sit in the same room with new colleagues. He used excessive amounts of perfume to mask the body odour and always tried to sit near open windows. He had low self-esteem and suff ered multiple episodes of depression and anxiety attacks. Although his brother and father had the same symptoms they never discussed this with family members. The diagnosis was unclear and a Google search led us to a probable diagnosis of trimethylaminuria. Trimethylaminuria is characterised by the presence of large amounts of trimethylamine in body fl uids, owing to a defi ciency of the enzyme fl avin-containing monooxygenase 3 (FMO3) in the liver, which oxidises the odorous trimethylamine (TMA) into its nonodorous N-oxide (TMA N-oxide; fi gure). The ratio of TMA N-oxide/(TMA+TMA-N-oxide) in our patient’s urine was less than 1 (normal >99) denoting that the odorous TMA concentration was very high. This fi nding was compatible with a diagnosis of trimethylaminuria. Confi rmation was obtained by genetic analysis, which showed that the patient was compound heterozygous: four FMO3 mutations were found on two alleles [E308G,E158K,Q385P (c.1154A>C), E305X]. His mother carried the known severe mutation E305X, his father the other three, and his brother had the same genotype. We treated our patient with a strict diet and when last seen in May, 2010, his odour had improved and his self confi dence had increased. TMA-N-oxide is present in considerable amounts in marine fi sh and after death is converted to TMA by bacteria resulting in the characteristic smell of rotting fi sh. Trimethylaminuria is diagnosed by determining the ratio of TMA-N-oxide to (TMA+TMA-N-oxide) in urine. There are several types, of which the primary genetic form is the most common. The variant allele containing the mu tations [E308G,E158K] is associated with mild fi sh odour syndrome. The novel Q385P mutation might add to the severity of the disease. The only possible treatment is a diet low on choline and lecithin (avoidance of fi sh, eggs, legumes, and off al), which are precursors of trimethylamine. The fi rst description of fi sh odour syndrome dates back to Mahabharata, the Sanskrit epic of the Bharata dynasty. In Shakespeare’s The Tempest the slave Caliban, who probably had a fi sh odour syndrome, is mentioned: “What have we here? a man or a fi sh? dead or alive? A fi sh: he smells like a fi sh; a very ancient and fi sh-like smell; a kind of not the newest Poor-John” (a salted and dried hake is called a “poor John”). People with trimethylaminuria often have a powerful aroma of rotting fi sh, which is destructive to their social life, and they have higher rates of depression and suicide. In one study 1% of a British-Caucasian cohort was heterozygous for one of the mutations that can cause trimethylaminuria. Since patients are often ashamed, they do not easily present to a doctor. It is important to draw attention to this condition so that more people can be successfully treated.