A cytological screening program for the early detection and prevention of cervical cancer was started in the county of Malmöhus, southern Sweden in 1967. The incidence of invasive cervical cancer among women properly attending the screening program has been reduced over the 10 years to about one quarter of the incidence before the initiation of the program. Figures calculated from the whole female population, however, did not indicate a significant decrease in incidence. Adequate individual follow-up combined with computerized administrative programs and population registers seem to be necessary whenever the effects of a mass screening program are evaluated.