Renal malposition of Mobin-Uddin filters is exceptional. In the few cases published the patients were asymptomatic; they were operated upon for fear of an eventual nephrotic syndrome. In the case reported here, however, an emergency operation was imposed by the severity of the clinical signs. Since the operation is simple and since renal thrombosis might have important immediate or late consequences, all filters malpositioned in the renal vein should be surgically removed.