Some studies have indicated that breastfeeding is protective against Sudden Infant Death Syndrome (SIDS).  Two of these studies are:

A recent study indicated that exclusive breastfeeding was protective against SIDS. Consistent with an effect mediated through the prevention of gastrointestinal and /or respiratory disease. Hoffman, H.J., "Risk Factors for SIDS: Results of the National Institute of Child Health and Human Development SIDS Cooperative Epidemiologic Study". Ann NY ACAD Sci, 1988.

Not exclusively breastfeeding at discharge from an obstetric hospital at any stage of the infant's life was associated with an increased risk of SIDS. Mitchell, A. "Results from the First Year of The New Zealand Count Death Study". N.Z. Med A, 1991; 104:71-76