Objectives: This study aims to determine femoral cartilage thickness using ultrasonography in familial Mediterranean fever (FMF). Patients and methods: A total of 45 patients (16 males, 29 females; mean age: 38.5±9.1 years; range, 24 to 49 years) with the diagnosis of FMF and 31 healthy individuals (6 males, 25 females; mean age: 37.0±8.7 years; range, 25 to 47 years) between January 2016 and July 2016 were included in this study. Clinical data and demographic characteristics of the patients were recorded. All FMF cases in the study were in remission with colchicine treatment. The thickness of femoral cartilage in both knees were evaluated using ultrasonography. Three measurements (mid-point) were taken from both knees (at the medial/lateral femoral condyles and intercondylar area). Results: Ultrasonographical measurements revealed that cartilage measurements of FMF patients were significantly thinner at both the medial/lateral femoral condyles and intercondylar area on the right knee and at the medial/lateral femoral condyles on the left knee (p<0.001). The cartilage measurements in FMF patients were significantly thinner at the intercondylar area on left knee, compared to those in controls (p=0.023). Conclusion: Our study showed decreased femoral cartilage thickness in FMF patients. These findings indicate that even if these patients do not have an attack, they may have subacute and chronic arthritis in their joints, and their femoral cartilage thickness can be affected.