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Patient: Male, 74 Final Diagnosis: Rheumatoid arthritis (RA) Symptoms: Joint pain • joint swelling Medication: — Clinical Procedure: — Specialty: Rheumatology Objective: Rare co-existance of disease or pathology Background: Paget disease is a primary bone disease with adjacent joint involvement that commonly presents with degenerative arthritis. Rheumatoid arthritis (RA) is mainly an articular disease with symmetric synovitis of the peripheral joints and usually does not involve the lumbar spine. The coexistence of Rheumatoid Arthritis and Paget disease of bone is infrequently reported in the literature. Case Report: A 74-year-old African-American man was referred for the investigation of symmetrical polyarthritis, left upper arm joint deformity, and low back pain. X-ray imaging of the hands demonstrated bilateral erosions and deformities of the carpal bones, and the left arm showed mixed lytic and sclerotic bone lesions. Laboratory tests showed high levels of rheumatoid factor and alkaline phosphatase. Analysis of the synovial fluid from the left knee was consistent with inflammatory joint disease. Conclusions: The purpose of this article is to present the diagnostic challenges associated with the rare coexistence of Rheumatoid Arthritis and Paget disease of bone in the same patient. The correct diagnosis of both diseases has important consequences in preventing treatment delay.