Introduction: Surveys that collect information on injuries often focus on the single "most serious" event to help limit recall error and reduce survey length. However, this can mask less serious injuries and result in biased incidence estimates for specific injury subcategories. Methods: Data from the 2002 Health Behaviour in School-aged Children (HBSC) survey and from the Canadian Hospitals Injury Reporting and Prevention Program (CHIRPP) were used to compare estimates of sports injury incidence in Canadian children. Results: HBSC data indicate that 6.7% of children report sustaining a sports injury that required an emergency department (ED) visit. However, details were only collected on a child's "most serious" injury, so children who had multiple injuries requiring an ED visit may have had sports injuries that went unreported. The rate of 6.7% can be seen to be an underestimate by as much as 4.3%. Corresponding CHIRPP surveillance data indicate an incidence of 9.9%. Potential masking bias is also highlighted in our analysis of injuries attended by other health care providers. Conclusion: The "one most serious injury" line of questioning induces potentially substantial masking bias in the estimation of sports injury incidence, which limits researchers' ability to quantify the burden of sports injury. Longer survey recall periods naturally lead to greater masking. The design of future surveys should take these issues into account. In order to accurately inform policy decisions and the direction of future research, researchers must be aware of these limitations.