Complement 9 deficiency is the most common complement deficiency in Japan, but it is rare in western countries. Because of Korea's geographical proximity to Japan, C9 deficiency in Korea has also been assumed to be common although this has never before been proven. We investigated complement deficiency in the serum samples of 6,159 Korean hospital outpatients. The deficiency was screened by a sensitive hemolytic assay and was confirmed by immunoassay of each complement component. Three C9-deficient individuals were found, giving an incidence of 0.049%, which is lower than that in Japan but still a considerable figure. Complement deficiencies other than that of C9 were not detected in this study. It is therefore necessary to consider the possibility of C9 deficiency in the interpretation of unexpectedly low complement-mediated hemolytic activity in East Asians.