This epidemiological study was carried out to investigate the sources of infection and modes of transmission of an outbreak of rubella that occurred among male high school students in Kyongju in march, 1996. 770 male students(286 third grade, 262 second grade, 222 first grade students) were selected as the study subjects. A questionnaire was completed by the subjects. The anti-Rubella antibodies IgM and IgG were examined and tested by ELISA on all sera from 770 students and the environmental status of class rooms, the dormitory conditions and study rooms were tested. The positive IgM result rate was 17.8% and when the IgM negative was combined with IgG positive the results was 74.9%. IgM IgG negative rates were 7.3%, however, out of a total 770 students. Amongst the older students, the IgM positive rate showed an increase(p<0.01). Out of 137 cases(with a cummulative incidence rate of 71.0%), there were 97 apparant cases(cummulative incidence rate 50.3%) and 40 inapparent cases(cummulative incidence rate 20.7%). Again, the cummulative incidence rate showed an increase in the older students(p<0.04). In the apparent cases, major symptoms included eruption(96.9%), fever(85.6%), lymphadenitis(82.5%), generalized aches and muscle tenderness(76.3%), and a sore throat(61.9%). These symptoms lasted anywhere from 2 degrees =/ 13 days, the average duration being 4 days in length. Initially, there were 9 people on third grade and suspected origin was from more than 2 external sources. It progressed from the upper grades down to the lower grades. In dormitory students, the cummulative incidence ratio was 1.70 and 1.78 amongst study room students.