Objectives - to determine the incidence of inflammatory bowel diseases (ulcerative colitis and Crohn's disease) in the population of Osijek-Baranja County in a unique fifteen-year period from 2000 to 2014 and at five-year intervals (2000-2004; 2005-2009; 2010. -2014), identify the most significant demographic, environmental and social factors that contributed to the incidence of inflammatory bowel disease in the population of Osijek-Baranja County and examine the characteristics of certain phenotypes of inflammatory bowel disease (ulcerative colitis, Crohn's disease) and response to therapy in the population of Osijek-Baranja County. Study Design - Retrospective cohort study Material and Methods - Respondents are adult patients diagnosed with inflammatory bowel disease from the Osijek-Baranja County who were diagnosed with the disease in the period from 2000 to 2014. The study was conducted by analyzing the medical records of KBC Osijek and OB Našice as the only two centers that provide secondary health care in the Osijek-Baranja County. Each of the respondents was additionally contacted by the researcher and asked to participate in the research and provide additional information about their disease according to the defined protocol, which will supplement the data obtained from the medical documentation of the respondents. Data were collected from medical records to determine: incidence, prevalence, phenotype, remission, relapse, therapeutic approaches, and treatment of inflammatory bowel disease. In order to monitor the phenotypic characteristics, the degree of disease activity, relapse, remission and the success of therapy, hematological laboratory tests were performed. Results - The increase in the incidence of inflammatory bowel diseases in Osijek-Baranja County is associated with demographic and social factors that affect the development of the clinical picture of the disease such as diatery habits, education, exposure to stressors. Clinical manifestations are in accordance with the available literature - E2 phenotype is the most common clinical presentation of ulcerative colitis, L3 phenotype is the most common clinical presentation of Crohn's disease, anemia and hepatobiliary complications are most common extraintestinal presentations of inflammatory bowel disease, surgery is common in Crohn's disease. Conclusion - The results of this study confirmed the hypothesis - the increase in the incidence of inflammatory bowel disease in Osijek-Baranja County is associated with demographic and social factors influencing the development of the clinical picture of the disease. The results of clinical presentations of inflammatory bowel disease confirm previous findings and are consistent with other studies.