-Context -The incidence and populational prevalence of inflammatory bowel diseases, hitherto unknown in Brazil, were estimated for a region in the Midwest of São Paulo State, Brazil. Methods -Using a sequential registry of 115 adult patients (>15 years old) with inflammatory bowel diseases -exclusively residing in the studied region with 533,508 inhabitants (2005) and attended at the reference hospital during a 20 year interval (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) -were estimated, in four consecutives periods of 5 years each, the incidences according to gender, type of the disease and the prevalence of these diseases, and its inner-relations evaluated by the Poisson regression model. Results -The inflammatory bowel diseases in the studied region predominated among young, white race and people living in urban area, and the incidence on the female population rose during this period. The incidence of ulcerative colitis were higher than Crohn's disease and non-classified colitis, and showed a progressive increase in the first three periods with a decrease in the last one (2001)(2002)(2003)(2004)(2005), where the observed rates of ulcerative colitis, Crohn's disease and non-classified colitis were 4.48, 3.50 and 1.75 cases/100,000 inhabitants, with prevalence of 22.61, 14.81, 5.65, 2.14 cases/100,000 inhabitants for total inflammatory bowel diseases. Conclusion -The inflammatory bowel diseases incidence in the studied area was as low as in other countries of Latin America and smaller than that found in countries of South Europe. The crescent prevalence justifies the policies to adequate medical cares for inflammatory bowel diseases patients in this area. HEADINGS -Inflammatory bowel disease, epidemiology. Colitis, ulcerative, epidemiology. Crohn disease, epidemiology. São Paulo (Brasil).