Estimates of the prevalence and incidence of knee osteoarthritis increase with age and are higher for women. Other differences in such estimates arise due to methodologic variables, including radiographic techniques. The risk for knee osteoarthritis increases with increasing body weight, long-term occupational joint stresses, joint trauma, and probably higher bone mass. Weight restriction and cigarette smoking are associated with reduced risk of knee osteoarthritis. The relationships among these risk factors (eg, bone mass, physical activity, weight, and smoking) and other influences (eg, genetics) remain to be classified.