Objective.To compare the ability to detect calcium pyrophosphate (CPP) crystals deposition (CPPD) in knee cartilage by ultrasonography (US) and radiography.Methods.Patients with knee effusion were consecutively included and underwent radiography and US evaluation of knees. Diagnosis of CPPD was made by the identification of CPP crystals. Two blinded rheumatologists performed US assessment.Results.We included 51 patients (25 with CPPD). US revealed hyperechoic spots in all 25 patients with CPPD (sensitivity 100%, specificity 92.3%), whereas radiography revealed CPPD in 16 (sensitivity 64%, specificity 100%; p < 0.0001).Conclusion.US of knees is more sensitive than radiography for CPPD diagnosis.