During a 6 years period, 25 newborns admitted to a neonatal intensive care unit died suddenly in the first year of life. The risk of sudden death in this specific population is 5% and reaches 7.4% in the low birth weight population (less than or equal to 2,500 g). In 12 infants, the sudden death was partially explained; in 7 infants, the sudden death remained unexplained on the basis of clinical and necropsic findings; in 6 infants, the clinical data did not justify the sudden death but necropsies were not performed. A comparison with matched living control infants showed that there was a significant increase of minor malformations of the branchial arch system and that in the sudden infant death group, threatening premature labour was more frequent during the mothers' pregnancies.