Paget's disease of bone (commonly known as Paget's disease or historically, osteitis deformans) is a condition involving cellular remodeling and deformity of one or more bones. The affected bones show signs of dysregulated bone remodeling at the microscopic level, specifically excessive bone breakdown and subsequent disorganized new bone formation. These structural changes cause the bone to weaken, which may result in deformity, pain, fracture, or arthritis of associated joints. The exact cause is unknown.

The pathogenesis of Paget's disease is described in 3 stages:
1) Osteoclastic activity = lytic
2) Mixed osteoclastic – osteoblastic activity
3) Osteoblastic activity = sclerotic

Initially, there is a marked increase in the rate of bone resorption in localized areas, caused by large and numerous osteoclasts. These localized areas of pathological destruction of bone tissue (osteolysis) are seen radiologically as an advancing lytic wedge in long bones or the skull. The osteolysis is followed by a compensatory increase in bone formation induced by the bone forming cells, called osteoblasts, that are recruited to the area. This is associated with accelerated deposition of lamellar bone in a disorganized fashion. This intense cellular activity produces a chaotic picture of trabecular bone ("mosaic" pattern), rather than the normal linear lamellar pattern.

Although there is no cure for Paget's disease, medications (bisphosphonates and calcitonin) can help control the disorder and lessen pain and other symptoms.

Complications include highoutput heart failure due to AV shunts and increased risk of osteosarcoma.