Objective: We evaluated the association of parity to both risk of knee replacement (KR) and knee osteoarthritis (OA). Design: The NIH-funded Multicenter Osteoarthritis Study (MOST) is a longitudinal observational study of persons age 50-79 years with either symptomatic knee OA or at elevated risk of disease. Baseline and 30-month knee radiographic OA (ROA) was defined as Kellgren/Lawrence (K/L) grade ≥2 or KR. Women were grouped based by number of births: 0; 1 (reference group); 2; 3; 4; and 5 or more. We examined the relation of parity to the incidence over 30 months of ROA and KR using a Poisson regression model. Generalized estimating equations (GEE) were used to control for correlation between two knees within a subject. We adjusted for age, BMI, race, education, occupation, baseline estrogen use, clinical site, injury, and for KR analyses WOMAC pain and use of pain medication. Results: Among 1618 women who reported parity information, mean age was 62.6 years, mean BMI 30.7kg/m2, mean WOMAC pain subscale score 3.7 at baseline. There were 115 KRs and 134 cases of incident knee ROA over 30 months. The relative risk of incident KR was 2.7 times as high (95% CI: 1.0, 7.3) and relative risk of incident knee ROA was 2.6 times as high (95% CI: 1.2, 5.3) among women with five to 12 children compared with those with one birth. Conclusion: Parity in women at risk for OA is associated with both incident ROA and KR, particularly for those with more than four children. © 2013 Osteoarthritis Research Society International.