Familial hypertrophic cardiomyopathy (FHC) is a genetic disorder resulting from mutations in genes encoding sarcomeric proteins 1, 2. This typically induces hyperdynamic ejection 3, impaired relaxation, delayed early filling 4, myocyte disarray and fibrosis, and increased chamber end-systolic stiffness 5, 6. To better understand the disease pathogenesis, early (primary) abnormalities must be distinguished from evolving responses to the genetic defect. We did in vivo analysis using a mouse model of FHC with an Arg403Gln α-cardiac myosin heavy chain missense mutation 7, and used newly developed methods for assessing in situ pressure–volume relations 8. Hearts of young mutant mice (6 weeks old), which show no chamber morphologic or gross histologic abnormalities, had altered contraction kinetics, with considerably delayed pressure relaxation and chamber filling, yet accelerated systolic pressure rise. Older mutant mice (20 weeks old), which develop fiber disarray and fibrosis, had diastolic and systolic kinetic changes similar to if not slightly less than those of younger mice. However, the hearts of older mutant mice also showed hyperdynamic contraction, with increased end-systolic chamber stiffness, outflow tract pressure gradients and a lower cardiac index due to reduced chamber filling; all'hallmarks' of human disease. These data provide new insights into the temporal evolution of FHC. Such data may help direct new therapeutic strategies to diminish disease progression.