Hyperinsulinism of infancy is a major cause of persistent hypoglycaemia in the newborn period. Transient mild self-limiting hyperinsulinaemia and hypoglycaemia have been described in neonates born to mothers taking ritodrine therapy for premature labour. Ritodrine crosses the placental barrier and enters the fetal circulation readily but the mechanism of how it causes hyperinsulinaemia and hypoglycaemia is unclear. We report the case of severe prolonged hyperinsulinaemic hypoglycamia in a neonate born to a mother taking ritodrine therapy from 16 weeks' gestation for preterm labour. The hyperinsulinaemic hypoglycaemia was managed with oral nifedipine as diazoxide was contraindicated due to fluid overload. Possible mechanisms of ritodrine-induced hypo glycaemia and insulin secretion are discussed. Copyright (C) 2005 S. Karger AG, Basel.