Résumé : Background: In the Sudden Infant Death Syndrome (SIDS), a medullary serotonergic network deficiency theory has been proposed, amongst many other hypotheses. The correlation between serotonergic neurons or dendritic spines in the brainstem of SIDS and sleep apnea was investigated here. Materials and methods: Twenty-seven thousand infants were studied prospectively to characterize their sleep-wake behavior. Of these, 38 infants died under 6 months of age, including 26 cases of SIDS. The frequency and duration of sleep apnea were analyzed. Brainstem material was collected and immunohistochemistry for tryptophan hydroxylase (TrypH) carried out. The density of TrypH-positive neurons was measured quantitatively. Correlation analyses were carried out between the TrypH-associated pathological data and the physiological data of sleep apnea. Results: One significant positive correlation between the density of TrypH-positive neurons in the dorsal raphe nucleus of the midbrain and the duration of central apnea (p = 0.027) was found in SIDS victims. Conclusions: Some of serotonergic facts could be involved in the pathophysiology of SIDS. © 2003 Elsevier Ireland Ltd. All rights reserved.