The caffeine effects on chromosomal aberration frequency and mean G2 duration were studied in human lymphocytes in vitro from three age groups of normal donors (I: 1-5 years old; II: 30-40 years old; III: 60-70 years old). Under control conditions, the three age groups showed a similar frequency of chromosomal aberrations. All three age groups exhibited a linear dose response for aberrations with caffeine treatments. However, lymphocytes from aged individuals (groups II and III) showed higher chromosomal aberration frequencies and longer G2 duration than cells from young individuals (group I). The caffeine effect in reducing G2 length was rather similar in every age group. The reversion of caffeine effects by adenosine or niacinamide in lymphocytes from older individuals was higher than in cells from group I. The different caffeine effects and G2 values between lymphocytes from old and young individuals are most likely due to a higher number of DNA lesions reaching G2 phase and/or a decrease of the G2 repair capability of lymphocytes from older individuals.