Cervical smears were reviewed from 62 women who developed squamous carcinoma of the cervix up to 18 years later. The findings indicate that the prevention of cervical cancer by screening depends very largely on the detection of severe dyskaryosis. In this series there was no evidence that mild dyskaryosis was a forerunner of invasive disease. Cytology during the evolution of squamous carcinoma is not characterised by a dyskaryosis which progressively increases in severity. Instead the findings support new concepts that cervical cancer generally arises from an aggressive CIN 3 lesion widely present in the cervix, and in our series, established years before invasion occurs. It would be more useful to report cytology as showing either a low or high grade abnormality rather than distinguishing between different degrees of dyskaryosis.