OBJECTIVES: Cost predictions are useful to identify patients with special needs early. Crohn’s disease (CD) and ulcerative colitis (UC) are complex conditions which are classified as Inflammatory Bowel Diseases (IBD). Disease onset in early adulthood leads to a long period of healthcare utilisation. The goal of this study was to determine the effect of disease activity and disease history on overall medical costs and to determine whether additional factors predict costs for CD and UC. METHODS: Medical history, demographic information and healthcare use (4 weeks) were reported by German IBD-Association members. Healthcare costs were calculated using national sources. Disease activity was determined using a German IBD Index. Patients classified their disease history as constantly-active, intermittently-active or in remission. Other factors evaluated were education, marital status, health insurance, an IBD-related operation (at least 3 months ago), smoking status and employment status. Missing values for determinants were imputed using the Markov-chain-Monte-Carlo method. Cost determinants were analysed using a gamma regression model, adjusting for age, sex, disease duration and for previous colectomy in UC patients. Costs of CD and UC were analysed separately. RESULTS: The 4-week mean direct medical costs were 424.44€ for CD (n=241) and 365.79€ for UC (n=238). Mean disease activity (CD: 3.3 UC: 3.2) was similar between the groups and 46% of both CD and UC subjects reported disease history as in remission. Significant predictors of medical costs (p<0.05) were disease activity, disease history for both groups, age and employment status for CD subjects and previous colectomy for UC subjects. CONCLUSIONS: Our analyses show that medical costs of patients with IBD can in part be predicted. However, predictors differ between CD and UC patients: Disease activity and disease history are the most important cost predictors, while age and employment status are only important cost predictors for CD patients.