A 10-year-old boy had a two-year history of a rotten fish odor, noted particularly in the summer after exercise. The odor was partially controlled by frequent bathing. Dietary history was unremarkable except for large daily milk intake. Milk elimination resulted in a prompt disappearance of the odor. Studies of the patient's urine after choline loading established the diagnosis of trimethylaminuria. Restriction of dietary milk as well as foods containing choline and trimethylamine oxide eliminated the offensive odor.