The policy concerning organization of screening for cervical cancer has varied in the Danish counties. Organized programmes started in the municipality of Frederiksberg in 1962, in the municipality of Copenhagen and the county of Maribo in 1967, and in the country of Copenhagen in 1968. On the other hand organized screening programmes have never existed in the counties of Vestsjaelland, Viborg, Ringkøbing and Ribe. We have used this "natural experiment" to study the influence of organized screening on the incidence and mortality of cervical cancer in Denmark. Women aged 30-59 years during the period 1968-87 are included in the study. The effect of organized screening has been estimated employing multiplicative Poisson models, where age, period, region and organized screening have been considered. Women living in counties where organized screening started three or more years ago have a statistically significantly reduced risk of cervical cancer. The relative risk for the incidence in this group is 0.77 (95% confidence interval 0.69-0.86), and the relative risk for the mortality is 0.75 (95% confidence interval 0.61-0.92). Only a part of the decrease in the incidence and mortality of cervical cancer which can be attributed to the organized screening activity. The additional decrease can be attributed to the considerable unorganized screening activity during the past 20 years, and to changes in exposure to risk factors.