Holocarboxylase synthetase deficiency is typically a biotin responsive disorder that presents with lactic acidosis, tachypnea, temperature instability, and shock in neonates (Briones et al. 1989 and Fuchshuber et al. 1992). The primary defect in cases studied to date appears to be the decreased affinity of HCS for its substrate, biotin (Gompertz et al. 1971). Supplemental biotin can provide sufficient substrate to increase HCS enzymatic function and thereby permit biotinylation of the four carboxylase apoenzymes (Briones et al. 1989). We report an infant with HCS deficiency who presented with lactic acidosis, shock, and hypertonia. Subependymal cysts were identified on cranial ultrasound and subsequently confirmed by MRI. Six months following biotin supplementation, she is developmentally normal and MRI of the brain shows complete resolution of the cysts.