Holocarboxylase synthetase (HCS) is an enzyme that catalyzes biotin incorporation into carboxylases, and its deficiency causes biotin-responsive multiple carboxylase deficiency. We report a patient who had his first episode at 32 months of age. The main clinical findings were a characteristic rash, projectile vomiting, progressive consciousness loss, organophosphate order, and hypotension. Laboratory examinations showed metabolic acidosis with ketolactic acidosis, hyperammonemia, and urine organic acid profile suggestive of a biotin utilization abnormality consistent with multiple carboxylase deficiency. Nucleotide sequence analysis of the biotinidase gene of the patient revealed negative finding, however, analysis of HCS gene found a homozygous 1809C->T (R508W) mutation. R508W is a rare mutation in Taiwanese HCS deficiency patients, which is associated with the late-onset phenotype. The patient responded dramatically to biotin, and has remained normal growth and development during more than three years of follow-up. Therefore, a high index of suspicion for timely diagnosis and treatment could prevent severe complications.