PURPOSE: Cardiopulmonary rehabilitation, which often follows major acute cardiac events, is traditionally focused on aerobic exercise and has been associated with decreased morbidity and mortality. Its benefit among cardiac surgery patients is less clear, as is the role of resistance-based exercise programs and their sex-specific effects. This study seeks to evaluate the safety and feasibility of a 12-week resistance training program in patients post cardiac surgery through a sex-specific lens. METHODS: We conducted a nonrandomized feasibility trial with a 12-week strength training exercise intervention. The primary outcome was safety and feasibility. Secondary outcomes included changes in strength, endurance, and functional capacity; and sex differences among these. Adult participants post open-heart surgery who had completed traditional cardiac rehabilitation were consented. Both patients who completed (cases) or did not complete (controls) a tailored 12-week resistance training program underwent comprehensive assessment of physiologic and physical fitness measures pre- and postintervention. FINDINGS: Nine participants enrolled in the trial, including 6 in the intervention arm (median age 61 years; 67% male) and 3 in the control arm (median age 66 years; 67% male). No serious adverse events were noted, indicating safety of the intervention. Participants completed a mean of 34.8/36 (96.7%) of sessions, indicating the feasibility of the program. Although not powered for statistical significance, patients experienced positive trends of improvement in measures of hand grip strength, endurance, and functional capacity with the intervention. When stratified, females experienced greater gains than males in these measures. IMPLICATIONS: This proof-of-concept study found that resistance-based exercise after cardiac surgery is well tolerated and feasible. Although all patients experienced improvements in exercise parameters, females reported greater relative improvement than males.