Rubella has been a notifiable disease in Iceland since 1888. In this century rubella epidemics of increasing size have occurred at intervals of 5-10 years. The disease has spread throughout the country. About 75% of the patients were below 15 years of age at the time of the last two epidemics. Females account for 51-61 per cent of the reported cases. In this first study of the sero-epidemiology of rubella in Iceland, epidemiological data and blood samples were collected from 1464 women from 13 locations. The group of 1464 women was further divided into 12 age-groups (range 5-49 years), so as to make correlations with the last six epidemics. The immunity status was determined by measuring HI-antibodies in serum. 67.8 percent of the sera were positive (HI-titre greater than 20). 81.1 per cent of women of child-bearing age (16-42 ys.) were seropositive. Two of the age-groups have gone through only one epidemic '63-'64. Children who were then 1-3 ys. old have an immunity ratio of 46.1 per cent, this ratio being 73.3 per cent in the case of children who had attained school age. The immunity ratio is slightly lower among subjects from rural districts than among those from urban districts. Most of the women have caught the disease at the time of the first or second epidemic to occur in their lifetime. 11.6 per cent of persons giving a positive history were seronegative, 52.1 per cent of seropositive subjects gave a positive history. According to population statistics and the serological results it is estimated that appr. 1 out of 7 cases was reported in the epidemic '63-'64.