Context: Addressing safe-play knowledge and player aggression could potentially improve ice hockey sport safety.Objectives: To compare (1) safe-play knowledge and aggression between male and female adolescent ice hockey players and (2) head-impact frequency and severity between players with high and low levels of safe-play knowledge and aggression during practices and games.Design: Cohort study. Main Outcome Measure(s): One-way analyses of variance were used to compare safe play knowledge and aggression between sexes. Players were categorized as having high or low safe-play knowledge and aggression using a median split. A 2 3 2 mixed-model analysis of variance was used to compare headimpact frequency, and random-intercept general linear models were used to compare head-impact severity between groups (high, low) and event types (practice, game).Results: Boys (5.8 of 7 total; 95% confidence interval [CI] ¼ 5.3, 6.3) had a trend toward better safe-play knowledge compared with girls (4.9 of 7 total; 95% CI ¼ 3.9, 5. Conclusions: Coaches and sports medicine professionals should ensure that athletes of all levels, ages, and sexes have full knowledge of safe play and should consider aggression interventions for reducing head-impact severity among aggressive players during practice.

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