Background and aims: Increasing incidence in Inflammatory Bowel Disease (IBD) has been suggested. Recent data on population based incidence rates within Europe are however scarce. Primary aim was to investigate prospectively the incidence of IBD within a well-defined geographical and administrative area of the Netherlands, the South Limburg IBD registry. Secondary aims were to study the duration of symptoms before diagnosis (tag time) and seasonal influences on the incidence of IBD. Methods: The incidence was examined using standardized registration of all newly diagnosed IBD patients, between 1-1-1991 and 1-1-2003. Medical records were reviewed to verify the diagnosis. At inclusion, diagnostic tag time was registered in months. Results: Age standardized incidence rates per 100,000 person-years (p-y) were: Crohn's Disease, mate 4.84, female 7.58; Ulcerative Colitis, mate 8.51, female 6.92; and Indeterminate Colitis, mate 1.05, female 0.93. Incidence rates did not significantly changes over time in either Crohn's Disease, Ulcerative Colitis or Indeterminate Colitis. Lag time was 5 (0-360) months in Crohn's Disease, 3.0 (0-480) months in Ulcerative Colitis and 3.0 (0-180) months in Indeterminate Colitis. Lag time was not significantly different between the periods 1991-1993 and 2000-2002, and no statistical differences in the onset of symptoms per calendar month or season were found. Conclusions: Our results, from the South Limburg region (the Netherlands), show no significant change in incidence rates of IBD. The incidence found is relatively high compared to other European countries. Lag time did not change during the study period, and seasonal influence of incidence rates could not be confirmed. Crown Published by Elsevier B.V. on behalf of European Crohn's and Colitis Organisation. All rights reserved.