Tumor-specific TP53 mutations are detectable in the blood plasma of tumor patients. Mutations of the TP53 tumor suppressor gene are risk factors for tumor progression. The objective of this work is to compare the presence of TP53 mutations in plasma-DNA before and after tumor treatment with the status of this gene in the tumor tissue sample. DNA was extracted from plasma samples of 25 patients with gastrointestinal tumors, and from paraffin-embedded tumor tissues from the same patients. Temperature gradient gel electrophoresis (TGGE) was performed for mutation screening of exons 5-8 of GC-clamped polymerase chain reaction products. Mutation-positive and wildtype gel bands from TGGE were cut and reamplified for fluorescence-labeled sequence analysis. The results of several mutation analyses were correlated with analysis of p53 autoantibodies in the same plasma. Mutation frequency (one or several mutations per sample) was 7.1% in blood plasma of tumor-free patients, 87.0% in tumor tissues, 78.6% in plasma before tumor treatment, and 36.8% after treatment. Fifteen of 22 mutations in tumor tissues of 13 patients also were detected in the same exons of plasma before treatment (68.2%). Mutations in plasma after treatment (2-684 days) were the same in 6 of 30 cases of tissue mutations only. Six of seven patients with mutations after treatment in their plasma had metastases. One patient was p53 autoantibody negative, but has a terminator mutation of codon 196 in tissue and in posttreatment plasma as well. Genetic analysis of plasma in tumor patients should be further developed, as it might be of prognostic value.