The R403Q mutation in β‐myosin heavy chain was the first mutation to be identified as responsible for familial hypertrophic cardiomyopathy (FHC). In spite of extensive work on the functional sequelae of this mutation, the mechanism by which the mutant protein causes the disease has not been definitely identified. Here we directly compare contraction and relaxation mechanics of single myofibrils from left ventricular samples of one patient carrying the R403Q mutation to those from a healthy control heart. Tension generation and relaxation following sudden increase and decrease in [Ca²⁺] were much faster in the R403Q myofibrils with relaxation rates being the most affected parameters. The results show that the R403Q mutation leads to an apparent gain of protein function but a greater energetic cost of tension generation. Increased energy cost of tension generation may be central to the FHC disease process, help explain some unresolved clinical observations, and carry significant therapeutic implications.