27 children with early onset (less than 4 years) diabetes (EOD), 24 with late onset (greater than 4.0 years) diabetes (LOD), and 30 sibling controls were compared in their performance on tests of intellectual functioning and school achievement. The results indicated that children with EOD, particularly girls, scored lower than the other groups of diabetic children and siblings on tests of visuospatial but not verbal ability. Many of the children with EOD were also having difficulty at school, and a number were currently receiving special education. Diabetic children with earlier onset had more hypoglycemic convulsions than those with later onset. Regression analyses revealed that duration of illness, age of onset, and hypoglycemic convulsions significantly predicted spatial ability.