Introduction: The body mass gain is conditioned by lifestyle, as well as many environmental and genetic factors. Recent studies suggest that renin-angiotensin system (RAS) plays a fundamental role in process of growth and differentiation of adipocytes through the acting of angiotensin II and seems to be a significant factor in excessive weight gain development. The purpose of this study was to determine the frequency and significance of insertion/deletion polymorphism (I/D) of the ACE gene in pregnant women with excessive weight gain. Materials and methods: The examined group consisted of 212 pregnant women, including 107 women with normal (ΔBMI≤5) and 105 women with excessive weight gain (ΔBMI>5). Genomic DNA was extracted from venous blood. The I/D polymorphism of ACE gene was determined by polymerase chain reaction (PCR). Results: During the course of the study we did not observe the statistically significant higher frequency of ACE genotypes in any of the two investigated groups of women with normal and excessive weight gain. Nevertheless, an overrepresentation of II genotype frequency in group with excessive weight gain has been observed (33,3 vs 21,5%, p=ns). The same findings were visible as far as the frequency of I allele in group with excessive weight gain was concerned (55,2 vs 45,8%, p=ns). The frequency of observed genotypes was in agreement with Hardy-Weinberg equilibrium. Conclusions: Although overrepresentation of II genotype and I allele in the group of pregnant women with excessive weight gain (ΔBMI>5) has been observed, a close correlation between II genotype and higher risk of overweight could be not indicated (due to the lack of significant difference). The results should be confirmed in a more numerous group of pregnant women. At this stage the results of the study did not suggested the presence of association of I/D polymorphism of ACE gene with weight gain in investigated group of pregnant women.