BACKGROUND Congenital hyperinsulinism (CHI) can present with considerable clinical heterogeneity which may be due to differences in the underlying genetic etiology. We present two siblings with hyperinsulinaemic hypoglycaemia (HH) and marked clinical heterogeneity caused by compound heterozygosity for the same two novel ABCC8 mutations. CASE PRESENTATION The index patient is a 3-year-old boy with hypoglycaemic episodes presenting on the first day of life. HH was diagnosed and treatment with intravenous glucose and diazoxide was initiated. Currently he has normal physical and neurological development, with occasional hypoglycaemic episodes detected following continuous fasting on treatment with diazoxide. The first-born 8-year-old sibling experienced severe postnatal hypoglycaemia, generalised seizures and severe brain damage despite diazoxide treatment. The latter was stopped at 6-months of age with no further registered hypoglycaemia. Genetic testing showed that both children were compound heterozygotes for two novel ABCC8 missense mutations p.I60N (c.179T>A) and p.G1555V (c.4664G>T). CONCLUSIONS These ABCC8 missense mutations warrant further studies mainly because of the variable clinical presentation and treatment response.