A 10-month-old boy presented with dermatitis and alopecia and became severely hypotonic. Screening for urinary organic acids revealed a large quantity of 3-hydroxyisovaleric acid and raised levels of beta-methylcrotonylglycine and 3-hydroxypropionate. Activities of propionyl CoA carboxylase, beta-methylcrotonyl CoA carboxylase, and pyruvate carboxylase in cultured fibroblasts were normal. Treatment with oral biotin resulted in a dramatic clinical improvement, which might therefore suggest a defect in biotin absorption or transport.