BACKGROUND: Prior studies show that men are more likely than women to defer essential care. Enrollment in high-deductible health plans (HDHPs) could exacerbate this tendency, but sex-specific responses to HDHPs have not been assessed. We measured the impact of an HDHP separately for men and women. METHODS: Controlled longitudinal difference-in-differences analysis of low, intermediate, and high severity emergency department (ED) visits and hospitalizations among 6007 men and 6530 women for 1 year before and up to 2 years after their employers mandated a switch from a traditional health maintenance organization plan to an HDHP, compared with contemporaneous controls (18,433 men and 19,178 women) who remained in an health maintenance organization plan. RESULTS: In the year following transition to an HDHP, men substantially reduced ED visits at all severity levels relative to controls (changes in low, intermediate, and high severity visits of -21.5% [-37.9 to -5.2], -21.6% [-37.4 to -5.7], and -34.4% [-62.1 to -6.7], respectively). Female HDHP members selectively reduced low severity emergency visits (-26.9% [-40.8 to -13.0]) while preserving intermediate and high severity visits. Male HDHP members also experienced a 24.2% [-45.3 to -3.1] relative decline in hospitalizations in year 1, followed by a 30.1% [2.1 to 58.1] relative increase in hospitalizations between years 1 and 2. CONCLUSIONS: Initial across-the-board reductions in ED and hospital care followed by increased hospitalizations imply that men may have foregone needed care following an HDHP transition. Clinicians caring for patients with HDHPs should be aware of sex differences in response to benefit design.