In a survey of 1781 patients who had mild dyskaryosis in a cervical smear taken between 1965 and 1984 invasive cancer occurred in 10 women. In four cancer was diagnosed soon after presentation, and in three it developed some years after default from follow up. Invasion occurred in one patient during cytological surveillance and in a further two after referral for colposcopic supervision. A poor correlation was found between a single cervical smear showing mild dyskaryosis and biopsy results. This was, however, improved by a series of smears. During initial surveillance cervical smear results reverted to normal in 46% of our patients within two years. During longer term follow up none of these patients developed invasive cancer, and life table analysis showed that three quarters had not relapsed after 14 years. We also found no evidence to suggest that preinvasive disease is more rapidly progressive in younger women. Our results indicate that cytological surveillance is acceptably safe provided that biopsy is advised if dyskaryosis persists.