Examination of the records of hospital admissions and all deaths among the 40,544 Sheffield children who were aged less than two years in the period 1975-79 revealed 475 children admitted with their first febrile convulsion and 50 children who presented as cot deaths. Rectal temperatures taken before refrigeration were traced for 24 of the latter. 10 of these 24 children had temperatures in excess of 38 degrees C and 5 were greater than 40 degrees C. There was a transition in age at presentation between the two groups, with most cot deaths occurring before six months of age and febrile convulsions after that age. No time-based clustering between the date of presentation with cot death or febrile convulsion was found to suggest a single aetiological agent. It is suggested that febrile convulsions and some inexplicable sudden infant deaths may be equivalent responses to a febrile stimulus, but with a different outcome because of the difference in age of occurrence. A common mechanism could be thermolabile syncope which, in the susceptible younger infant, leads to irreversible cerebral ischaemia. The rectal temperature should be taken as soon as possible in all cot deaths. An aberration of a vagally mediated reflex may be responsible for some cot deaths.