Diagnostic methods for the incompetence of the internal cervical os are described. The incompetence of the internal cervical os when there is no pregnancy is especially dealt with. For the diagnosis five tests are used (measurement of the uterine cavity and palpation of the cervical channel, Palmer test, Bergman-Svennerund test cervicohysterography, and X-ray cervicometry). Only in strictly indicated cases should cerclage be applied. In cases with an undeveloped clinical picture the operation could be recommended twelfth week of pregnancy. With an appropriate selection of patients and an operation after the twelfth week of pregnancy a success of 82% was achieved at the university hospital in Ljubljana. The duration of the sick-leave after the operation is also indicated.