Twenty-four fatal cases of echo 11 infection in the eleven years 1968-78 are presented. All were children, and could be divided into two groups according to age at death and clinical presentation. The first group comprised 12 babies who died aged between 5 and 11 days after a short illness characterised by collapse, acidosis, and bleeding. At necropsy there was evidence of disseminated intravascular coagulation with haemorrhage into many organs including the renal medulla, suprarenal glands, gastrointestinal tract, and central nervous system. Six cases showed hepatic necrosis which was massive in three. Virus was present in many tissues. Infection was probably acquired from the mothers at delivery in 3 cases. Low maternal neutralising antibody titres and prematurity were thought to be adverse factors in the outcome. The second group consisted of 12 children aged between 9 weeks and 4 years 10 months who died suddenly. Pathological findings included upper respiratory tract infection, pneumonia, encephalitis, and gastroenteritis. Six of this group had been classified as 'cot deaths'. The role of echo 11 in the death of some of these older children is unknown. This report shows the danger of echo 11 to neonates, especially if unprotected by maternal antibody.