Fish-odor syndrome: An inborn error of metabolism that is associated with an offensive body odor whose scent is similar to the smell of rotting fish. Fish-odor syndrome is due to the excessive excretion of trimethylamine (TMA) in urine, sweat, and breath. This syndrome is caused by a mutation in the gene for the enzyme flavin-containing monooxygenase-3 (FMO3). Dietary modifications to reduce the consumption of triethylamine precursors may provide some relief of symptoms. Also known as trimethylaminuria.