Microsatellite instability (MSI) is observed in a subset of endometrial cancers (ECs) and is attributed to defects in mismatch repair. Mismatch repair deficiency allows for accumulation of mutations in the coding repeats of key target genes, which may be involved in the initiation and progression of MSI+ EC. We examined genes implicated in DNA repair pathways in 38 MSI-high (MSI-H), 10 MSI-low, 25 microsatellite stable ECs, and a selected panel of associated premalignant hyperplasias. Genetic alterations were correlated to histopathological data, including tumor grade and stage. Somatic frameshift mutations were observed in hMLH3, hMSH3, hMSH6, CHK1, and BAX genes in MSI-H endometrial hyperplasias and cancers, whereas mutations in ATR and CDC25C were observed only in MSI-H ECs. Increased mutation frequency in DNA damage response pathway genes including ATR, CHK1, and BAX demonstrated a significant trend with advancing tumor grade (P < 0.05). Our observations of the same mutations at short coding mononucleotide repeats in both premalignant lesions and tumors and association of increased frequency of mutation accumulation with advancing tumor grade suggest that these alterations may play a role in the development and progression of MSI+ EC.