The purpose of this study is to determine the prevalence of Brachyspira hyodysenteriae from 43 swine farms with diarrhea or a history of diarrhea in Gyeongbuk province by PCR. The overall herd prevalence of B hyodysenteriae was 37.2%(16/43), and the average prevalence of B hyodysenteriae among all sampled pigs was 10.8%(50/462). Positive herds for B hyodysenteriae were distributed throughout Gyeongbuk province from 14.3～50%. Herd size had a effect on the frequency of B hyodysenteriae. The frequency of B hyodysenteriae in herds with less than 1,000 pigs was 47.4%, that of herds between 1,001 and 2,000 pigs was 41.7%. Also, the frequency of B hyodysenteriae in herds with more than 2,000 pigs was 16.7%.