This study is for patients with hypertrophic cardiomyopathy (HCM). HCM is a condition where the heart muscle becomes abnormally thickened, which can sometimes block the blood flow out of the heart and results in the heart muscle working harder to pump blood to the body. Participants who have completed participation in a previous HCM study investigating the study drug, called aficamten (CK-3773274), will be eligible to participate in this study.

The study is done to collect long-term safety and tolerability data, including assessments of cardiac structure and function during chronic dosing with aficamten. Aficamten is a tablet taken by mouth. This is an open label study (the participants and study team will know the dose of aficamten taken at any given time). If your screening results show you are eligible to continue in the study, you will visit the research site for the "first dosing day" (Day 1), followed by visits at Weeks 2, 4, 6, 12, then every 12 weeks thereafter. Study related procedures include blood work, echocardiograms (ultrasound test of the heart), electrocardiogram (recording of heart's electrical activity), physical exams, and questionnaires. Risks associated with this study include shortness of breath, nausea, diarrhea, headaches and dizziness.