An epidemiological research of multiple sclerosis (MS) in Istria, Yugoslavia, was made in the period of 1980-1981. After examining all the sources of health care information, 125 potential MS patients were found in the investigated area. According to the diagnostic criteria by Schumacher et al., 47 affected were recognized and accepted as clinically definite MS patients. The MS prevalence rate in Istria on March 31st, 1981 amounted to 25.0/10(5) inhabitants (CI: 19.9-38.9). Such rates classify Istria in the middle between the medium and high risk zones for the disease in Europe and in the world. The onset age of MS in Istria was about 30 years, the female/male sex ratio was 2.13. The average duration of MS in Istria up to the prevalence day was 16.5 years. The average annual incidence rate was 1.5/10(5) inhabitants.