From 30 to 35% of idiocy and 8% of imbecility were due to birth injury in 100 autopsied cases. 3 types of pathology were encountered in the brains of 24 traumatic mental defectives studied. Mantle sclerosis or granular atrophy is the result of subcortical encephalo-malacia with reactive brain gliosis and mesenchymal organization which follows traumatic cortical destruction. Cystic degeneration[long dash]diffuse[long dash]occurs only in infancy and appears to be related to long-lasting asphyxiation during birth. Patchy cortical devastation encountered in 16 cases suggests an origin due to a combination of venous hemorrhages and anoxia. Some of the most important factors contributing to birth injuries are prematurity, breech presentation and prolonged labor.