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Our goals were to answer 2 questions: (1) Is the presentation of early-onset inflammatory bowel disease (IBD) similar to typical adolescent-onset IBD? (2) Is there variability in familial aggregation in childhood IBD?The phenotype of IBD in children under 5 years of age (early-onset) is poorly defined. Clinical and genetic studies of IBD, however, generally assume the phenotype to be homogenous throughout childhood.We analyzed data from 413 consecutive pediatric IBD outpatients attending our center between 1995 and 2000. Disease type, anatomic distribution, and family history were compared between children presenting before (early-onset) and after the age of five (5 to 15 y).Disease presentation was predominantly colonic in early-onset IBD, most patients presenting with ulcerative colitis (UC). Isolated colonic disease was most frequent in early-onset Crohn disease (colonic 76.5%, ileocolic 24%) compared with ileocolic disease (ileocolic 45.5%, colonic 26%, ileal 19.4%, proximal 6.3%) in the older age group. First-degree family history was highest in early-onset UC 26% versus 11% in the older UC group.We describe a distinct phenotype of early childhood onset IBD, with a strikingly high familial aggregation in UC and greater tendency to present with colonic disease. As more genetic heterogeneity is identified in IBD, careful definition of phenotype is required to identify further susceptibility genes. The early-onset form of UC presents an ideal group for further genetic analysis. These phenotype differences also suggest that treatment and outcome may vary in early-onset childhood IBD; prospective studies are required to confirm this.