The best results in screening for subclinical cancer of the cervix are given by cytological studies. The authors in reviewing their experience in this method of screening studied the result of 15,000 smears which gave a diagnosis of 37 cases of intra-epithelial carcinoma of the cervix, which means 1 case of carcinoma in situ diagnosed in every 400 smears. Although the technique is very simple it has to be carried out according to strict criteria at the time of taking the smear, of fixing it immediately and of staining it. There is a group of patients who are at high risk about the age of forty, when there is a marked influence due to parity. But only a systematic policy will bring about diagnosis of pre-invasive carcinoma in cases where the cervix is clinically healthy. The cytology is usually characteristic, though the diagnosis may be difficult during pregnancy, or when hormone contraception is being used or when there is a trichomonas infection present. This explains why we find false positives in 0.03 per cent of cases in this study and false negatives in 0.03 per cent of cases. Their rarity means that the method is 99.94 per cent reliable. Finally, the cervical smear gives an opportunity for studying the vaginal microbial flora as well as the cyto-hormonal state.