BACKGROUND Knee and hip injuries have been linked with osteoarthritis in cross-sectional and case-control studies, but few prospective studies have examined the relation between injuries in young adults and risk for later osteoarthritis. OBJECTIVE To prospectively examine the relation between joint injury and incident knee and hip osteoarthritis. METHODS Prospective cohort study. METHODS Johns Hopkins Precursors Study. METHODS 1321 former medical students. METHODS Injury status at cohort entry was recorded when the mean age of participants was 22 years. Injury during follow-up and incident osteoarthritis were determined by using self-administered questionnaires. Osteoarthritis was confirmed by symptoms and radiographic findings. RESULTS Over a median follow-up of 36 years, 141 participants reported joint injuries (knee alone [n = 111], hip alone [n = 16], or knee and hip [n = 14]) and 96 developed osteoarthritis (knee alone [n = 64], hip alone [n = 27], or knee and hip [n = 5]). The cumulative incidence of knee osteoarthritis by 65 years of age was 13.9% in participants who had a knee injury during adolescence and young adulthood and 6.0% in those who did not (P = 0.0045) (relative risk, 2.95 [95% CI, 1.35 to 6.45]). Joint injury at cohort entry or during follow-up substantially increased the risk for subsequent osteoarthritis at that site (relative risk, 5.17 [CI, 3.07 to 8.71] and 3.50 [CI, 0.84 to 14.69] for knee and hip, respectively). Results were similar for persons with osteoarthritis confirmed by radiographs and symptoms. CONCLUSIONS Young adults with knee injuries are at considerably increased risk for osteoarthritis later in life and should be targeted in the primary prevention of osteoarthritis.