The histologic criteria used to diagnose ulcerative colitis in colonic mucosal biopsies have been established for many years and include crypt architectural distortion, plasmacellular infiltrates, and neutrophils in the crypt epithelium and lumen. In several recent studies, it has been noted that colonic mucosal biopsies from children presenting with ulcerative colitis show fewer histologic abnormalities at initial presentation, especially less architectural distortion, than do biopsies from adults. In this study, colonic mucosal biopsies taken at the time of presentation of ulcerative colitis in 15 adults and 25 children were examined blindly by two pathologists. All biopsies were taken prior to the initiation of therapy. Twelve children were between 1 and 10 years of age, and 13 children were between the ages of 11 and 17 years. All patients had at least 1 year of follow-up, with clinical and pathologic confirmation of the diagnosis of ulcerative colitis. Five separate histologic features that are characteristic of ulcerative colitis were scored on mucosal biopsies. Children < or = 10 years of age had significantly less crypt branching, plasma cells in the lamina propria, cryptitis, crypt abscesses, and epithelial injury than adults (P values ranging from < 0.0001 to 0.0032). Children between the ages of 11 and 17 years had less cryptitis, crypt abscesses, and epithelial injury than adults (P values ranging from 0.0001 to 0.007) but similar degrees of crypt architectural distortion and plasma cell infiltrates. For all histologic features examined except epithelial injury, the significant findings were due to differences in biopsies taken proximal to the rectum. No significant differences in histology scores were found in rectal biopsies between any age group, except for epithelial injury, which was significantly less in children < or = 10 years. The findings show for the first time that the perceived differences between adults and children with ulcerative colitis are largely due to a decrease in histologic features of colitis in children less than 10 years of age. As children approach adulthood, the degree of inflammation and architectural distortion seen is similar to that found in adults. However, rectal biopsies show similar degrees of colitis in all age groups.