Objective. To determine the relationship between quadriceps weakness and cartilage damage and bone marrow lesions (BMLs) in the patellofemoral joint (PFJ) and if this relationship is modified by patella alta. Methods. The Multicenter Osteoarthritis Study is a cohort study of persons ages 50-79 years with or at risk of knee osteoarthritis. Concentric knee extensor strength was measured using an isokinetic dynamometer. Patella alta was measured using the Insall-Salvati ratio (ISR) on the lateral radiograph, and cartilage damage and BMLs were graded on magnetic resonance imaging in the PFJ. We determined the association between quadriceps weakness with cartilage damage and BMLs in the PFJ among knees with (ISR ≥1.2) and without (ISR <1.2) patella alta using multiple binomial regression. Results. A total of 807 knees were studied (mean age 62 years, body mass index 30 kg/m 2 ISR 1.10) and 64% were from women. Compared with the knees in the highest strength tertile, those in the lowest had 10.2% (95% confidence interval [95% CI] 3, 18), 9.1% (95% CI 2, 16), and 7.1% (95% CI 1, 13) higher prevalence of lateral PFJ cartilage damage, medial PFJ cartilage damage, and lateral PFJ BMLs, respectively. The association between quadriceps weakness with cartilage damage and BMLs was not different between knees with and without patella alta in the lateral PFJ. Conclusion. Quadriceps weakness was associated with PFJ cartilage damage and BMLs. While both patella alta and quadriceps weakness were associated with PFJ damage, the combination of the two was not associated with more damage than either of these factors alone. © 2011, American College of Rheumatology.