Anomalies seem to be present in the brain of infants who died of sudden infant death syndrome. They imply dendritic synapses, myelinisation, some neurotransmitters, some peptides, their enzymes and receptors. More precisely, catecholaminergic systems seem to be abnormal. The changes were found mainly in brain stem, in vegetative central areas. These anomalies are probably maturational in nature. They could represent a fragility of vegetative control systems, especially cardiorespiratory and swallowing control systems. They probably do not explain, per se, the occurrence of death; they probably would need to be potentiated by additional etiological factors (environmental, viral, etc). These anomalies have to be considered as provisional and, in our opinion, need confirmation. Apart from the brain stem, other areas are under investigation, particularly cerebral structures implied in temperature control, sleep-wakefulness control and hemispheric structures.