Serum orosomucoid was compared with clinical activity, routine laboratory tests, intestinal protein loss and albumin and Ig[immunoglobulin]G turnover in 22 cases of ulcerative colitis. Serum orosomucoid was well correlated with clinical activity, but Hb and leukocyte counts were not. A significant correlation was present between serum orosomucoid and intestinal protein loss (gastrointestinal 59Fe dextran clearance), serum albumin, fractional catabolic rates of albumin, and IgG and IgG synthesis rate. No correlation was found between serum orosomucoid and albumin synthesis rate or serum IgG. Serum orosomucoid probably is a highly reliable indicator of disease activity in ulcerative colitis.