Mass screening programs for cancer of the cervix were introduced at different times and to different extents in the Scandinavian countries. Iceland and Finland introduced nationwide screening programs in the early 1960s. Ten years after their introduction, marked reductions in incidence and mortality of invasive cervical cancer were observed, seen specifically in the age groups to which screening was targeted. In Sweden and Denmark, mass screening was introduced on a county basis, and changes in incidence and death rates from cervical cancer were recorded which reflected the degree to which organized mass screening had been introduced. These data demonstrate that mass screening scheduled every two to five years can reduce the incidence of invasive cervical cancer by 80%. No evidence is available from Scandinavia that more frequent screening reduces the risk further.