Allelic imbalance and microsatellite instability in operating materials from 78 patients with gastric cancer was studied. Microsatellite polymorphism for 17p13.1 (TP53), 1p36.1 (RUNX3), 16p22 (CDH1), and MH (BAT26) was determined in tumor and adjacent (morphologically normal) tissues of gastric mucosa. The allelic imbalance of 17p13.3 (p = 0.0176) and 16p22 (p = 0.023) loci by two and more loci in a single sample (p = 0.0176), as well as microsatellite instability (p = 0.047), is observed significantly more frequently in intestinal types of tumors than in tumors of a diffuse type. During the comparison of clinical groups with different degrees of tumor-cell differentiation, it was demonstrated that allelic imbalance by 16p22 locus (p = 0.041) and by two and more loci in a single sample (p = 0.0057) is observed more frequently in highly differentiated or moderately differentiated tumors. We did not detect significant differences in the groups of patients with metastases (or without them) in regional lymphatic nodes with different localizations and at different stages of the tumor process.