Microsatellite DNA instability (MSI) is a consequence of disorder within mismatch repair genes coding DNA repair proteins, protecting the cell against replication errors. Their dysfunction leads to gathering of adverse mutations within a cell, which may result in its neoplastic transformation.The aim of the study was to analyse the occurrence of microsatellite DNA instability in polypoid adenomas of large intestine removed during endoscopic polypectomy.Material and methods. The study covered 97 patients (30 women and 67 men), who underwent colonoscopic polypectomy in Endoscopy Ward, 2nd Department of Surgery, Jagiellonian University, Medical College in Cracow, between 2004 and 2007. Sampled materials was verified histopathologically and genetic tests were performed with the use of ABI PRISM 310 sequenator, which enabled to diagnose microsatellite DNA instability. Between 90 and 360 days from the first colonoscopy, 86 (78.2%) patients underwent following endoscopic colonoscopy in order to search for new polyps of large intestine.Results. 130 polypoid adenomas were removed in 97 patients. Sigmoid colon was the most common location of lesions. Microsatellite DNA instability was diagnosed in 21 (16.6%) polyps, loss of heterozygosity was observed in 25 (19%) polyps. During control colonoscopy performed a year after the initial colonoscopy recurrence of polyps was stated in 15% of patients (7% of women and 15% of men). Microsatellite DNA instability was most commonly diagnosed for loci p53 di and DCC. Microsatellite DNA instability was more common in group of younger people and was related with larger polyps.Conclusions. Analysis of microsatellite DNA instability in polypoid adenomas of large intestine provides further essential information within the scope of studies on transformation of adenomas in malignant adenoma of large intestine.