Five thousand endodontic cases treated by three endodontists were reviewed to determine the frequency of the need to leave teeth opened for drainage. The subsequent treatment of these teeth is described. Of the 5,000 cases reviewed, 518 were left open at some point during treatment. Two hundred and eighty-one cases (54.2 percent) remained closed after the first closure. Four hundred and twenty-six (82.2 percent) of the cases opened were eventually completed without surgical intervention. The need to prescribe antibiotics and perform a trephination is also stated.