Purpose The L216R mutation, seen in individuals of Polynesian descent, is considered one of the most severe mutations associated with holocarboxylase synthetase (HLCS) deficiency and is regarded as being unresponsive to biotin. This report describes the presentation and outcome in two surviving siblings, homozygous for this highly lethal mutation. Methods and results Both cases had perinatal head imaging findings of brain hemorrhage and subependymal cysts. Both had metabolic decompensation within 24 h after birth consisting of metabolic acidosis, lactic acidosis, and thrombocytopenia. Biochemical profiles were consistent with HLCS deficiency, and genetic analysis confirmed homozygosity for the L216R mutation. After resolution of neonatal metabolic crisis, dosing of biotin was titrated on an outpatient basis to primarily control dermatitis. The eldest is currently on 1.2 g of oral biotin daily, well above any dose previously reported to treat HLCS deficiency. To date, neither patient has required hospital readmission for acute metabolic decompensation. At the age of 7, the eldest child is, to our knowledge, the oldest patient ever described in the literature who is homozygous for the L216R mutation. She has mild intellectual disability. Conclusion This report contrasts previous reports of poor outcomes and neonatal deaths in homozygous L216R patients. We also provide data on the potential upper tolerable limit of biotin. These cases suggest that the outcome of HCLS deficiency due to a homozygous L216R mutation, when diagnosed and treated early with high-level neonatal care and biotin, may not be as severe as previously reported.