OBJECTIVE: To determine whether infants at sleep in the prone side positions are at higher risk for an extreme cardiorespiratory event compared with infants at sleep in the supine position. STUDY DESIGN: We used a case-control study to compare sleep position, determined with an accelerometer, in 116 infants during an extreme cardiorespiratory event with that in 231 matched control subjects (2 per case) who did not experience any extreme events during monitoring. RESULTS: From calculation of adjusted ORs and 95% CIs, infants placed in the prone or side position were no more likely to experience an extreme cardiorespiratory event compared with infants at sleep in the supine position. We used conditional logistic regression to account for the matched design of the study and to adjust for potential confounders or effect-modifiers. CONCLUSION: These findings, coupled with our earlier observation that the peak incidence of severe cardiorespiratory events occurred before the peak incidence of sudden infant death syndrome, strongly suggest that the supine sleeping position decreases the risk of sudden infant death syndrome by mechanisms other than by decreasing extreme cardiorespiratory events detected by monitoring.