The aim of the present study was to assess serum levels of copper and zinc levels and erythrocytes Cu,Zn-SOD activity and to determine probable changes in gastric and colorectal precancerous diseases, benign breast diseases, gastric, colorectal and breast cancer. The study included 165 subjects with cancer, 348 subjects with precancerous (atrophic gastritis, gastric adenoma, colon adenoma, rectal adenoma) and/or benign diseases (weak dysplasia, severe dysplasia, fibroadenoma, cystic disease) and 161 randomly selected healthy controls. Our results suggest that while in gastric and colorectal cancer there were mostly increased copper levels, in breast cancer they were not changed. Zinc levels were weakly decreased in atrophic gastritis, gastric adenoma and breast cancer. There was a strong positive correlation between zinc levels and SOD activity in fibroadenoma and a weak positive correlation in colorectal adenoma and colorectal cancer without any correlation between SOD activity and copper in these groups. In gastric precancerous disease there was a positive correlation between SOD and copper. The results of this study suggest that serum trace element levels and activity of related enzymes might be different in various neoplastic processes. This variation in neoplastic processes might be influenced by other factors that have to be considered in complex relationships between the whole body and neoplastic cells.