Hypertrophic osteoarthropathy (HOA) is a syndrome characterized by digital clubbing and periostosis of the tubular bones. The clubbed portions of the fingers and toes consist of excessive collagen fiber deposition and accumulation of interstitial edema. Perivascular infiltrates of lymphocytes and vascular hyperplasia are responsible for thickening of the vessel walls. Subperiosteal, cancellous, and new bone formation exists along the distal diaphysis of tubular bones, which progresses proximally with time. Initially, excessive connective tissue and subperiosteal edema elevate the periosteum; then, new osteoid matrix is deposited beneath the periosteum Hypertrophy or bony overgrowth predominates in patients with lung cancer and hypertrophic pulmonary osteoarthropathy (HPOA), whereas, acroosteolysis predominates in patients with cyanotic congenital heart disease and HOA. Synovial involvement may occur with subperiosteal changes. Thickening of the sub synovial blood vessels and mild lining-layer hyperplasia may occur.