Myelodysplastic syndromes (MDS) may be associated with inflammatory bowel disease (IBD). We characterized the clinical features and outcomes of patients with concurrent IBD and MDS. Using a diagnostic index, we identified all patients with both IBD and MDS at our center between 1976 and 1997. We also calculated an incidence rate of MDS in IBD using population-based data from Olmsted County, Minnesota, between 1950 and 1993. Among -15,000 IBD patients seen, 25 (-0.17%) were diagnosed with MDS. Fourteen had Crohn's disease and I I had ulcerative colitis. The median age at diagnosis of IBD, particularly Crohn's disease, was higher than expected. Median age at diagnosis of MDS was typical. All but one ulcerative colitis patient was diagnosed before the diagnosis of MDS, while one-half of Crohn's disease patients were diagnosed with both ailments simultaneously. Five patients who had been diagnosed with IBD first were persistently anemic for at least 1 year prior to diagnosis of MDS. Two Crohn's ~ disease patients had received purine analogs in the past. Median follow-up after MDS diagnosis was 1 year. Seven patients died, including two who progressed to acute myeloid leukemia. The incidence rate of MDS in IBD based on Olmsted County data was 0 cases per 100,000 person-years (95% CI, 0-55.2). The seemingly high frequency of myelodysplastic syndromes in a large referral-based group of patients with IBD suggests an association; however, an increased risk of MDS was not observed in a small regional cohort of IBD patients. Patients with MDS are diagnosed with concurrent IBD at an age older than expected. Simultaneous diagnoses were made in one-half of Crohn's disease patients. MDS should be considered in the differential diagnosis of anemia in IBD patients.