The serum levels of five acute phase proteins (APP) were measured in 18 children with Crohn's Disease (CD) or ulcerative colitis (UC) and in two control groups. The levels of C-reactive protein, alpha 1-acid glycoprotein, alpha 1-antitrypsin, C9, and Factor B were significantly raised in patients with CD and UC with good separation from controls, but they were not entirely reliable used as screening tests unless used in combination. The levels of APP were monitored for periods varying from 18 to 28 months in each patient and found to reflect the disease activity in both CD and UC. On seven occasions the APP levels did not match the clinically assessed disease activity, but when the serum levels were related to outcome of the disease, C-reactive protein was found to be elevated--whether or not there were symptoms of the disease--in all patients who later had a relapse, while normal values were found in those who had a long remission. These results suggest that the estimation of Creative protein is of prognostic value and that its measurement is particularly useful in children with mild symptoms in whom disease activity and prognosis are difficult to assess.