A 16-week-old boy was found moribund in his crib. At autopsy widespread infiltration of multiple organs by lymphoblastic leukaemia (pre-B cell type) was found. Death was attributed to extensive myocardial infiltration by leukaemic cells. There was no associated intracranial haemorrhage. Home video footage taken on the day before death revealed a non-distressed and relatively normal-appearing infant. This case demonstrates an extremely rare cause of unexpected infant death and emphasizes the need for full autopsy examination in such cases. It also confirms that certain infants may have minimal or no symptoms of illness, even in the presence of severe, systemic, life-threatening disease.