Trimethylaminuria, also known as fish-odor syndrome, is a rare metabolic disorder characterized by the inability to convert trimethylamine, a strongly odorous compound, into its odorless derivative, trimethylamine N-oxide. The accumulation of trimethylamine in the body leads to a strong fish-like odor. Although clinically benign, the disorder has a significant psychosocial impact, affecting self-esteem, social interactions, and overall quality of life.This case describes the assessment of a female infant presenting to primary care with a recurrent fish-like body odor noticed shortly after the introduction of fish at 7 months of age. The evaluation included clinical history, physical examination, dietary trials conducted at home under medical guidance, and referral to a specialized Metabolic Diseases Clinic. The purpose was to clarify the cause of the odor and its reproducibility. A multidisciplinary approach involving primary care, nutrition and medical genetics allowed structured observation of symptom resolution with dietary avoidance and recurrence with reintroduction. Genetic analysis confirmed FMO3 deficiency, which encodes the enzyme flavin-containing monooxygenase 3. Measurable outcomes consisted of complete symptom remission after dietary modification and successful implementation of a long-term management plan.Unlike most reported cases, which are diagnosed later in childhood or adulthood, this case highlights an unusually early clinical suspicion and confirms the value of primary care in recognizing subtle signs of trimethylaminuria, that is rarely described in the literature.This case emphasizes the importance of early clinical suspicion and dietary evaluation in infants with unexplained body odor. The role of Family Medicine is also important to manage psychosocial impact of this condition within the family and community throughout life.This case illustrates the diagnostic challenge of trimethylaminuria in infancy, particularly when symptoms are subtle and triggered by early dietary changes. It reinforces the need to consider metabolic causes in atypical body odor and highlights the value of coordinated care in achieving timely diagnosis.This case emphasizes the importance of early clinical suspicion and dietary evaluation in infants with unexplained body odor, encouraging greater awareness in primary care and supporting earlier referral pathways to improve long-term management.