Baseline frequency of chromosomal aberrations was investigated in lymphocyte chromosomes of 60 healthy Korean individuals who were selected randomly each of 10 males and females from aged 40s, 50s, and 60s groups, respectively. A total of 6000 metaphase cells were screened for chromosomal aberrations. The mean frequency of chromosomal aberrations was 3.35%. The frequency of chromosomal aberrations was also analyzed with respect to the age, sex, and smoker. However, no significant difference was found among them except for smoker in dicentric chromosome which was higher in smokers (0.12%) than in non-smokers (0.07%).