Pachydermoperiostosis (PDP) is characterized by clubbing fingers, furrowing of the facial skin, and periosteal hypertrophy. We report a case of a patient with PDP associated with severe arthritis of the knee and ankle joints. His serum C-reactive protein (CRP) levels were increased, and an analysis of serum and synovial fluid showed high levels of interleukin-6. These findings mean that there is some difficulty in distinguishing the disease from rheumatoid arthritis. While treatments such as nonsteroidal antiinflammatory drugs, steroids, and colchicine were not particularly effective, the severe arthralgia was gradually relieved over a few years.Key words Arthritis · Clubbing fingers · Pachydermoperiostosis (PDP) Pachydermoperiostosis (PDP) was first described in 1935. 1 The symptoms were digital clubbing, periostosis, and hypertrophic skin changes. Although the disease is sometimes a complication of arthritis, few abnormal inflammatory signs have been demonstrated in serum and synovial fluids. It has been reported that the arthralgia in PDP does not originate from synovitis, but rather is caused by active inflammation of the periosteum. 2 However, some cases show swelling of the joints with synovitis and joint effusion, suggesting nonspecific arthritis. In this study, we report a case of a patient with PDP complicated by severe arthritis, and describe the inflammatory markers in the serum and synovial fluid of the patient.An 18-year-old man who had been affected with severe pain in both knee and ankle joints for 3 months visited the hospital. There was no relevant family history. On physical examination, clubbing of the fingers and toes was found, and there was marked furrowing of the skin on the scalp (cutis verticis gyrata) and face, as well as seborrhea on the face (Fig. 1). These abnormal findings had developed from about 17 years old. Swelling and local joint heat were seen in both knees and ankles. Radiological examination showed that periostosis of the diaphysis of the radius, ulna, tibia, and fibula was present, while no abnormal findings were shown in the knee or ankle joints or in the hands (Fig. 2). A chest X-ray film was normal. At the first examination, his hemoglobin was 14.7 g/dl, leukocyte count 8.4 ϫ 10 3 /mm 3 , and platelets 3.85 ϫ 10 5 /mm 3 . Serum calcium, phosphorus, alkaline phosphatase, and uric acid were normal. Rheumatoid factors, antinuclear factors, and lupus erythematosus cells were negative, and C-reactive protein (CRP) was 2.33 mg/dl (normal Ͻ0.23 mg/dl). Growth hormone level in serum was 2.1 ng/ml (normal Ͻ5 ng/ml). Because of these characteristic findings, we diagnosed this patient as a complete type pachydermoperiostosis. Despite treatment with a nonsteroidal anti-inflammatory drug (NSAID), his arthralgia did not improve. Three months later, we aspirated a small amount of joint effusion from the knee, which was clear and yellowish. We then measured interleukin-6 (IL-6) levels in serum and synovial fluid with an enzyme-linked immunosorbent assay (ELISA) kit (BioSource Internatio...