The protein mass of group II phospholipase A2 (PLA2) levels were immunochemically measured in sera of 43 patients with Crohn's disease (CD), 43 patients with ulcerative colitis (UC), and 35 healthy volunteers, and compared with PLA2 enzymatic activity. Serum immunoreactive group II PLA2 (IR-PLA2 II) levels of active CD patients were significantly higher than those of the controls and inactive CD patients. Serum IR-PLA2 II levels were also significantly greater in patients with severe UC and moderate UC than in the controls and patients with mild UC. Serum IR-PLA2 II levels were found to be closely correlated with serum PLA2 activity in patients with CD and UC. Serum IR-PLA2 II levels and PLA2 activity decreased by antiinflammatory treatment in six patients with CD and two with UC. These results suggest that serum IR-PLA2 II levels can be a useful parameter in the evaluation of disease activity of CD and UC, and that elevated PLA2 enzymatic activity in sera of patients with CD and UC is attributable to increased protein concentration of group II PLA2, in agreement with our previous results.