A study from the Osteoarthritis Initiative examined the effect of diabetes mellitus on the course of knee osteoarthritis in 2,483 participants aged 45 to 79 years during an eight-year follow-up. Results showed that patients with knee osteoarthritis and co-existing diabetes mellitus had significantly increased knee symptoms as measured by the WOMAC score (total score increased by 3.20 points, p = 0.004) compared to patients with osteoarthritis only. Specifically, pain (increase of 0.71 points, p = 0.003) and joint stiffness (increase of 2.26 points, p = 0.005) worsened. However, diabetes mellitus was not associated with increased pain severity in the short-term at seven days or at thirty days. The study provides evidence that diabetes mellitus is associated with worsening symptoms of osteoarthritis of the knee in the long term. These findings point to the need for increased attention in the treatment of patients who have both diseases at the same time.