The relationship of free radicals with some types of carcinomas is known, these studies have been performed on samples taken from the tumor tissue. In this study, the malondialdehyde levels in the serum of patients with skin cancer were studied and compared with a control group. The differences between malondialdehyde levels in these two groups were evaluated using Mann Whitney-U. There was a very statistically significant increase in malondialdehyde levels in serum of the skin cancer patients compared with the control group (p=0.0095, z=-2.59). Thus, increased serum malondialdehyde levels may be a sign of carcinoma.