Papanicolaou screening for cancer of the uterine cervix was introduced in Sweden in the late 1950's. Screening programmes covering the age groups 30-49 years were organized in various countries between 1965 and 1973. The approximate number of smears rose from 100 000 in 1960 to one million in 1970, in a female population of four million. Almost 60 000 cases of in situ carcinoma and 17 100 invasive carcinomas of the uterine cervix were registered in Sweden between 1958 and 1980. The age-standardized incidence of invasive carcinoma fell in this period by about 40%. Within the screened cohorts and age groups, the incidence was reduced by two-thirds and there was a parallel fall in mortality from the disease. At least part of these reductions seemed to be explained by the intensity of screening.