To our knowledge, the genotoxic effects of neoadjuvant chemoradiation therapy on molecular diagnostic testing results are unknown. However, if neoadjuvant treatments were to alter molecular test results, clinical decision-making could be misled. This raises questions about the appropriateness of using posttreatment tumor for testing. To address this, rectal adenocarcinomas both before and after neoadjuvant treatment were evaluated for alterations in KRAS and microsatellite instability (MSI) testing. Neoadjuvant chemoradiation therapy is common in this tumor type, and alterations in these 2 tests would significantly impact management. A total of 17 rectal adenocarcinoma patients with available pretreatment and posttreatment tumor were studied. MSI testing used the revised National Cancer Institute panel of 5 mononucleotide microsatellite repeats, comparing cancers with matched normal control tissues. KRAS codon 12-point and 13-point mutations were examined by polymerase chain reaction amplification and bidirectional sequencing. MSI and KRAS results were unchanged comparing rectal cancer tissue before and after chemoradiotherapy in all 17 patients (P=1.000; 95% CI: 0.3969-2.520). All 17 tumors (100%) were microsatellite stable. KRAS testing identified 12 (72%) wild-type tumors and 5 (28%) codon 12 or 13 mutant tumors with identical KRAS point mutations before and after treatment. The identified MSI and KRAS mutational prevalences parallel those reported in the rectal cancer literature. Neoadjuvant therapy did not alter KRAS codon 12 or 13 or MSI results in rectal adenocarcinoma, providing evidence that either pretreatment biopsy or posttreatment resection tissues are appropriate for testing.