Trimethylaminuria is a rare metabolic disorder that is associated with abnormal amounts of the dietary-derived trimethylamine. Excess unmetabolized trimethylamine in the urine, sweat and other body secretions confers a strong, foul body odor that can affect the individual's ability to work or engage in social activities. This review summarizes the biochemical aspects of the condition and the classification of the disorder into: 1) primary genetic form, 2) acquired form, 3) childhood forms, 4) transient form associated with menstruation, 5) precursor overload and 6) disease states. The genetic variability of the flavin-containing monooxygenase (form 3) that is responsible for detoxication and deodoration of trimethylamine is discussed and put in context with other variant forms of the flavin-containing monooxygenase (forms 1-5). The temporal-selective expression of flavin-containing monooxygenase forms 1 and 3 is discussed in terms of an explanation for childhood trimethylaminuria. Information as to whether variants of the flavin-containing monooxygenase form 3 contributes to hypertension and/or other diseases are presented. Discussion is provided outlining recent bioanalytical approaches to quantify urinary trimethylamine and trimethylamine N-oxide and plasma choline as well as data on self-reporting individuals tested for trimethylaminuria. Finally, trimethylaminuria treatment strategies and nutritional support are described including dietary sources of trimethylamine, vitamin supplementation and drug treatment and issues related to trimethylaminuria in pregnancy and lactation are discussed. The remarkable progress in the biochemical, genetic, clinical basis for understanding the trimethylaminuria condition is summarized and points to needs in the treatment of individuals suffering from trimethylaminuria.