Paget's disease or osteitis deformans is a bone metabolic disease characterized by an increased bone resorption followed by an increased but irregular bone formation. This results in a weakened, deformed bone and an increase bone mass with collagen fibers forming an irregular and pathologic mosaic instead of the parallel symmetry that characterizes healthy bone. It is considered as a non-neoplastic disorder that mimics a bone tumor. Its origin is still uncertain; however, it has been postulated that genetic or environmental factors are involved in its etiology. It rarely occurs in young patients and its prevalence increases with age. It usually affects both genders with a slight predominance of males. It may be symptomatic or asymptomatic depending on the bones involved, with the most common clinical manifestation being pain of the affected bone. Lesion distribution varies from monostotic involvement (25%) to generalized disease (75%). The most frequent complication is a fracture in a pathological area, while the most serious one is sarcomatous degeneration (approximately 1%; higher in the generalized form). The diagnosis is usually radiologic, by means of plain X-rays and biochemical markers such as alkaline phosphatase, among others, which is elevated in 85% of patients. Treatment of this disease consists of bisphosphonates, which have a proven efficacy and high remission rates. They are indicated in patients with clinical manifestations and in asymptomatic patients with evidence of disease activity. We report the case of a patient with Paget's disease in an infrequent location as is the patella.