Serological investigations for the leptospirosis on hospitalized patients in Choonchun Sungsim Hospital during the periods from August to November 1985 and 841 inhabitants of Kangwondo area including Choonchun, Choonsung, Inje, Chulwon, Hwachun, Gosung, Taibaik, Samchuk and Yangju area were carried out. 1. Among 58 hospitalized patients who were suspected as leptospirosis, 10 patients were detected to have antibody against Leptospira. All of positive sera had the highest antibody titer against serogroup Icterohemorrhagiae and most positive sera were also reactive to serogroup Australis and Canicola. Antibody titer of positive sera detected by microscopic agglutination(MA) test were ranging from 1 : 40 to 1 : 2,560. Antibody titer detected by ELISA method were higher than those detected by MAT(ELISA 1 : 400$sim$1 : 25,600) and IgM titer of positive sera were generally higher than IgG titer. 2, Of 841 inhabitants in 8 area of Kangwondo, 17 persons (2,02%) possessing antibody against Leptospira were detected by ELISA method, IgG titer in positive sera were generally higher than IgM titer. Persons possessing antibody to Leptospira were distributed in both sex and in various age group, and no significant regional and occupational fluctuations were obserbed.