The objective of this study was to determine femoral cartilage thickness (FCT) in patients with Klinefelter syndrome (KS), which is the most common sex chromosome disorder in men with varying degrees of androgen deficiency. This cross-sectional controlled study was conducted in a tertiary care center. Participants were 33 male patients (mean age = 36.2 years) with KS and 35 aged-matched healthy male controls (mean age = 32.9 years). Femoral condyle cartilage was measured by ultrasonography at three locations in each knee. Total testosterone, estradiol, sex hormone binding globulin, free androgen index, bioavailable testosterone, and albumin levels were measured. Femoral cartilage of patients at right and left knee lateral (3.03 and 2.99 mm), mid (3.81 and 3.74 mm), and medial (3.01 and 3.20 mm) were statistically thicker than in controls (right and left knee lateral 2.29 and 2.36 mm, mid 2.64 and 2.53 mm, medial 2.39 and 2.32 mm, respectively, p < .001). There was no significant correlation between FCT and sex hormones. In conclusion, patients with KS had thicker femoral cartilage. Relatively low testosterone levels in these patients and altered estrogen metabolism may hypothetically explain increased cartilage thickness in these patients.