In this study the levels of the mutant p53 protein in sera of patients with breast, lung and gastrointestinal tract cancer were determined in order to investigate the diagnostic usefulness of this parameter. The efficiency of the test was compared to the other tumor markers specific for each group. Serum p53 levels were determined in parallel with CEA and CA 19.9 for malignant gastrointestinal disorders. In patients with breast cancer, CEA and CA 15-3 serum levels were determined. Cyfra 21.1 serum levels were measured in patients with lung cancer. The positivity rates of elevated serum p53 values in the patients with gastrointestinal system and lung cancers were 43% and 37%, respectively. The frequency of the elevated values in the breast cancer group was 12%. Our observations and the data suggest that p53 might be one of the most promising molecular markers with regard to the understanding of the mechanism for tumorigenic changes and the biologic and clinical behaviour of the tumor.