OBJECTIVE: This study reported the case of a 3-year-old boy with biotinidase deficiency. The child was admitted with a 6 month history of alopecia and tetter and progressive lower limbs flaccidity for 3 months. Urinary organic acid analysis with gas chromatograph/mass spectrometry and biotinidase activity assay of blood confirmed the diagnosis of biotinidase deficiency. He presented with neurological abnormalities and dermatological lesions. Biotin supplementation (20 mg/d) led to a dramatic improvement of the symptoms. It was concluded that early diagnosis and biotin supplementation can greatly improve the outcome of patients.