The plasma levels of tyrosine and the lipid peroxidation intermediate malonic dialdehyde (MDA) were measured in children with pigmental nevus and melanoma. They were found to have antioxidative protective failure. The presence of pigmental nevus predetermines the high incidence of tyrosine metabolic abnormalities. In children with pigmental nevus and melanoma, there was an impairment in the correlation of the parameters of tyrosine metabolism, the rate of free radical lipid peroxidation processes, and the status of the antioxidative system. The findings suggest that children with pigmental nevus may be referred to as a high cancer risk group and monitoring of these parameters should be recommended at follow-up of pigmental nevi.