CONTEXT Sudden infant death syndrome (SIDS) is a leading cause of postneonatal infant mortality. Our previous meta-analyses showed that any breastfeeding is protective against SIDS with exclusive breastfeeding conferring a stronger effect. The duration of breastfeeding required to confer a protective effect is unknown. OBJECTIVE To assess the associations between breastfeeding duration and SIDS. DATA SOURCES Individual-level data from 8 case-control studies. STUDY SELECTION Case-control SIDS studies with breastfeeding data. DATA EXTRACTION Breastfeeding variables, demographic factors, and other potential confounders were identified. Individual-study and pooled analyses were performed. RESULTS A total of 2267 SIDS cases and 6837 control infants were included. In multivariable pooled analysis, breastfeeding for <2 months was not protective (adjusted odds ratio [aOR]: 0.91, 95% confidence interval [CI]: 0.68–1.22). Any breastfeeding ≥2 months was protective, with greater protection seen with increased duration (2–4 months: aOR: 0.60, 95% CI: 0.44– 0.82; 4–6 months: aOR: 0.40, 95% CI: 0.26–0.63; and >6 months: aOR: 0.36, 95% CI: 0.22– 0.61). Although exclusive breastfeeding for <2 months was not protective (aOR: 0.82, 95% CI: 0.59–1.14), longer periods were protective (2–4 months: aOR: 0.61, 95% CI: 0.42–0.87; 4–6 months: aOR: 0.46, 95% CI: 0.29–0.74). LIMITATIONS The variables collected in each study varied slightly, limiting our ability to include all studies in the analysis and control for all confounders. CONCLUSIONS Breastfeeding duration of at least 2 months was associated with half the risk of SIDS. Breastfeeding does not need to be exclusive to confer this protection.