Locking of the knee is an acute traumatic extension deficit with an inability to achieve full active or passive extension. True locking indicates a mechanical block while pseudo-locking indicates a functional block. Most cases of locking of the knee are due to meniscal and ligamentous injuries and loose bodies. Pigmented villo-nodular synovitis (PVNS) represents a rare but important cause of locking of the knee. Recognition of this cause will lead to early and definitive management.