A syndrome characterized by retropatellar or peripatellar pain resulting from physical and biochemical changes in the patellofemoral joint. The pain is most prominent when ascending or descending stairs, squatting, or sitting with flexed knees. There is a lack of consensus on the etiology and treatment. The syndrome is often confused with (or accompanied by) chondromalacia patellae, the latter describing a pathological condition of the cartilage and not a syndrome.