Pathologic synovial processes in the hip joint can cause characteristic extrinsic erosions of the femoral neck, which in extreme cases produce an "apple core" appearance. Nine such cases of synovial diseases, including synovial osteochondromatosis, pigmented villonodular synovitis, rheumatoid arthritis, and amyloidosis, that demonstrate this radiographic finding are presented. The anatomic relations of the hip joint that result in this appearance, differential diagnosis, and radiographic techniques useful in diagnosis are discussed.