Study Design Longitudinal observational study. Objectives To examine the relationship between severity of preseason hip, groin, and thigh (HGT) muscle injuries, and lumbopelvic muscle size, asymmetry, and function at the start and end of the preseason. Background In Australian Rules Football, HGT muscle injuries have the highest prevalence and incidence. Deficits within the lumbopelvic region, such as impaired muscle function and muscle asymmetry, could contribute to injuries in the preseason, and injury could, in turn, affect muscle size and function. Methods MRI examinations were performed on 47 male elite Australian Rules Football players at the start and at the end of the football preseason. The cross-sectional area (CSA) of multifidus, psoas major, and quadratus lumborum muscles was measured, as well as change in trunk CSA due to the function of voluntarily contracting the transversus abdominis muscle. Injuries occurring during each preseason training session were routinely recorded by the club's performance staff. Results Analysis of variance indicated that players with more severe preseason HGT injuries (more training sessions missed) had significantly smaller multifidus muscle CSA compared to players with no HGT injury (P = .006). No relationship was found for size or asymmetry of the quadratus lumborum or psoas major muscles, or ability to contract the transversus abdominis muscle through drawing in of the abdominal wall (P>.05). Small multifidus muscle size at L5 predicted 5 of 6 players who incurred a more severe HGT injury. Conclusions An association between multifidus muscle size (relative to age, height, and weight) and preseason injury suggests a way to identify players at risk of severe HGT injuries. This result needs to be replicated in a larger sample before resources are committed to intervention efforts. J Orthop Sports Phys Ther 2011;41(10):767–775, Epub 4 September 2011. doi:10.2519/jospt.2011.3755