Trimethylaminuria is a rare, autosomal recessive, metabolic disorder that results in accumulation of trimethylamine (TMA), which smells like rotten fish. The chemical is excreted in sweat and urine owing to a deficiency in the enzyme flavin monooxygenase 3 (FMO3). We report a case of trimethylaminuria in a 12-year-old girl. The patient failed treatment with diet and hygiene modification, but achieved symptomatic improvement after a four-month course of metronidazole.