Researchers tested the premise that the interaction between low back pain (LBP) and knee pain intensity contributes to the level of disability in individuals with knee osteoarthritis (OA). In patients with knee OA, LBP interacts with the intensity of knee pain and contributes to the level of disability. Coexisting LBP and knee pain have a stronger impact on disability level than either one alone. The potential deteriorative effects of LBP–knee interaction on disability were highlighted in the findings. Rather than targeting them separately, maximal treatment effects for disability might be achieved when LBP and knee pain are targeted simultaneously.