Reliable, valid measures are needed to assess one's sense of mastery, which has the potential for decreasing anxiety and depressive symptoms among patients with implantable cardioverter-defibrillators (ICDs). This study evaluates the reliability and validity of a measure of mastery, the Chronic Heart Failure Questionnaire (CHQ) mastery subscale. One hundred twenty-two (75% men, mean age 65 years) and 100 patients complete baseline and 12-month face-to-face interviews, respectively. The CHQ mastery subscale is found to have internal consistency reliability (Cronbach's alphas = .79, .84), and its validity is supported. Factor analysis yields a single robust factor. Differences in the CHQ mastery subscale scores by the New York Heart Association (NYHA) classes are found: Patients with NYHA Class III or IV have lower mastery than those with Class I or II. Baseline younger age and less frequent ICD shocks and lower mastery are significant predictors of respectively 12-month anxiety (R( 2) = .37) and depressive symptoms (R(2) = .45).The CHQ mastery subscale has demonstrated satisfactory reliability and validity in this sample.