This paper reviews the outcome in 12 children with hyperinsulinaemic hypoglycaemia who first developed symptoms between the ages of 2 and 8 months and who were treated with diazoxide (5 - 20 mg/kg/day) for 2-13 years. Two cases required subtotal pancreatectomy at the ages of 5 and 10 years because of recurrent hypoglycaemia and one girl with severe retardation died at the age of 6 years while still on diazoxide therapy. Two patients aged 3.5 and 9 years are still on treatment and in 7 cases diazoxide was discontinued between the ages of 2.5 and 14 years, indicating that spontaneous remission can be expected in a high proportion of children with post-neonatal hyperinsulinaemic hypoglycaemia. Of the 9 children who started diazoxide within 3 months of the onset of symptoms, 5 are of normal intelligence and 4 are moderately retarded (IQs 63-71). In 3 children diazoxide was started 8 months to 3 years after the onset of symptoms; two are retarded (IQs 60-70) and the third was severely retarded and died aged 6 years.