The incidence of malignant melanoma increases with age. One significiant effect of aging processes is anaccumulation of oxidative damage in the genetical material. In this study, the relationship between malignantmelanoma and damage in chromosomes and proliferative effectiveness of human peripheral lymphocytes wereinvestigated by the micronucleus (MN) technique. A total of 15 malignant melanoma patients and appropriatelymatching 15 healthy controls were involved in the study. MN frequencies and proliferative indexes (PI) afternon toxic levels of hydrogen peroxide treatment were also measured to determine damaging effect of oxidativestress in genome in addition to measuring the spontenous levels of micronuclei and PI. The patient group hada significantly higher rate of spontaneous MN than the control group (p<0.01). After treatment with H2O2, MNfrequencies in the patient group was significantly decreased (p<0.01) although there was no difference betweenthe treated and untreated results of control group (p=0.29). There was also difference (p<0.01) between the MNfrequencies of the patient and the control group either in the spontaneous levels or in the H2O2 treated groups.The same significant difference persisted when the PI values were compared between patient and control groups.Increase in the MN frequency in patients could mean the alterations in the chromosomal structure which maylead to the chromosome instability and therefore genetic susceptibility to cancer. This increased number ofmicronuclei can also be used for cytological marker in identifying high risk cases for malignant melanoma.