In order to establish the base line frequency of chromosome aberrations in normal Korean aged under 40 years old group, metaphase chromosomes of peripheral lymphocytes were analyzed. Sixty blood samples were drawn from each of 10 males and 10 females in each groups of teenager, twenties, and thirties, and whole blood were cultured. The values of chromosome aberrations obtained were analyzed with respect to age, sex and smoking habit. The overall mean frequencies of chromatid deletion, acentric chromosome, dicentric chromosome were 0.70, 0.43 and 0.10 %, respectively.