Arthritis tends to promote inactivity, and inactivity tends to promote an unhealthful constellation of blood abnormalities that increases the risk of heart attack and stroke. The hematology of inactivity comprises the following: low plasma volume, high hematocrit, high plasma fibrinogen, elevated blood viscosity, increased platelet aggregability, and diminished fibrinolysis. Regular exercise reverses all these adverse blood changes and, thereby, helps prevent heart attack and stroke. Simply put, exercise "improves" the blood, making it flow more easily and clot less readily. This "healthy hematology of exercisers" is one more reason why prudent exercise is as vital for patients with arthritis as it is for the rest of us.