The effectiveness of screening programmes based on cytological smears in reducing mortality from carcinoma of the cervix and the incidence of invasive disease is well established. But there is no evidence that the Papanicolau test has succeeded anywhere in complete eradication of this theoretically preventable disease. The major reason that many cervical cancer screening programmes have had little effect is that they fail to reach the women, particularly those aged over 40, who are most at risk of developing invasive carcinoma. Among women who are included in a programme, the principal failures are inadequate follow-up of abnormal smears, a long interval since the last smear, and false-negative smear results. Only when an acceptable participation level and follow-up system have been achieved should improvements in sensitivity assume. The potential failures of the system and the reasons for them and the remedies are described.