The relationship between the results of nine laboratory tests and inflammatory activity in the colon assessed macroscopically (macro) and histologically (hist) has been studied in 60 children and adolescents, 36 with ulcerative colitis (UC), and 24 with Crohn's colitis (CC). The mean duration of disease was 38 months. Eleven patients were newly diagnosed and not treated. Special attention was paid to severe and extensive colitis (SEC), in other words, moderate or severe inflammation in at least six of eight colon segments. A significant correlation was found between some laboratory parameters and the inflammatory activity of colonic mucosa. In UC, the following abnormal laboratory test results only occurred in SEC/macro: thrombocytosis, low serum albumin, and high serum orosomucoid. The combination of low serum iron and low or normal total iron binding capacity (TIBC) occurred in seven cases, six of whom had SEC/macro. Eleven of the 17 patients with SEC/macro had at least one of these four test results. SEC/hist was indicated only by thrombocytosis and high orosomucoid. In CC, a combination of low serum iron and low TIBC only occurred in SEC/macro. Thirteen of 36 patients with UC (36%) and four of 24 with CC (17%) had no abnormal test results.