The differential diagnosis between polymyalgia rheumatica and elderly-onset rheumatoid arthritis is difficult because these diseases share similar clinical findings, especially at onset. We report a case of elderly-onset rheumatoid arthritis that was distinguished from polymyalgia rheumatica with malignancy. A 77-year-old woman was admitted to our hospital because of pain and bilateral stiffness in her shoulders and her hips. Tests for rheumatoid factor and anti-cyclic citrullinated peptide antibody were negative. Bone erosions and joint space narrowing were not detected radiographically, and polymyalgia rheumatica was suspected. Her arthralgia disappeared with a few days after treatment with prednisolone (10 mg per day) was started, and ESR and CRP were normalized. Computed tomographic scanning of the chest showed a nodular lesion in the right lower lobe, and biopsy revealed lung cancer. Positron-emission tomography with 18F-fluorodeoxyglucose (FDG) performed before lung surgery showed increased uptake of FDG in the bilateral shoulder joints and wrist joints. Enhanced MRI showed synovitis and bone erosions in the right acromioclavicular joint and bilateral carpal bones and also radiographically bone erosions were seen in the carpometacarpal joint of the right thumb. Therefore, a diagnosis of elderly-onset rheumatoid arthritis was made. In patients with polymyalgia rheumatica, the detection of rheumatoid synovitis should be routinely evaluated.