The patient was a 64-year-old woman with a nearly 20-year history of sicca symptoms, having been given a diagnosis of primary Sjögren's syndrome. Three years previously, she experienced difficulty in walking up a slope and had leg malaise, which insidiously progressed to an inability to go up and down stairs. This disability brought her to our hospital, where her muscle strength was examined by manual muscle testing, and she was found to have reduced muscle strength in proximal muscles like the thigh muscles and the neck flexor muscles. Blood studies revealed elevated ESR, increased serum IgG, mildly increased myogenic enzymes, and positive results for anti-SS-A and -SS-B antibodies. MRI scans disclosed extensive muscle atrophy as well as fatty degeneration in the thigh. A biopsy of the quadriceps femoris muscle provided a diagnosis of myositis based on the finding of muscle fibers of unequal size, nuclear centralization, and inflammatory cell infiltration into muscle fibers. CD4-positive lymphocytes were the predominant inflammatory cells. We diagnosed the case as myositis in primary Sjögren's syndrome based on the clinical course and laboratory findings. She recovered well with steroid medication. It is noteworthy that myositis associated with primary Sjögren's syndrome presents with mild symptoms and unremarkable laboratory data but may run a chronic progressive course.