Diagnosis of localized pigmented villonodular synovitis is clinically difficult, and plain radiographs are usually normal. This article presents five patients with localized pigmented villonodular synovitis of the knee. Symptoms suggested a meniscal lesion in three patients and a loose body in two. Magnetic resonance imaging performed prior to arthroscopic resection revealed a preoperative diagnosis of a tumor in all five patients. Magnetic resonance imaging is a valuable clinical tool for the assessment of intra-articular soft-tissue tumors of the knee that may otherwise be misdiagnosed.