Background: Considering the increasing prevalence of knee joint pain and its complications and lack of adequate information about its causes this study was carried out to evaluate the incidence of knee joint osteoarthritis and its risk factors in patients referring to out patient departments of Kashan in 2004.

Materials and Methods: One thousand patients with knee joint pain enrolled to this cross sectional descriptive study. Knee joint X-ray was obtained from patients with signs and symptoms of osteoarthritis and those complying the criteria of American Society of Rheumatology (ACR), were selected and their demographic data including age, sex, weight, body mass index (BMI), history of trauma to knee, and smoking were recorded and analyzed with Chi square, and Fisher Exact tests.

Results: Three hundred and fifty patients (35%) had knee joint arthritis. From 350 patients 43 (12.3%) were male, and 307(87.7%) female. The most common age range of involvement in both males and females was 50-59. The most common BMI range was 30-40 Kg/m². Thirty seven (10.5%) of patients had history of severe trauma to knee joint.

Conclusion: This study showed that the most important risk factor for knee joint osteoarthritis in adult is heavy weight. Previous injuries to the knee joint and occupational factors are other important factors.