Circulating levels of vasoactive intestinal peptide (VIP) in plasma were measured in gauging activity in inflammatory bowel disease (IBD). One hundred-fifteen adult IBD patients were studied cross-sectionally and prospectively, 48 with ulcerative colitis (UC) and 67 with Crohn's disease (CD). Sequential samples of plasma were assayed for VIP by specific radioimmunoassay. Sixty males and 55 females, ranging in age from 22 to 76 years were studied over six months. The results revealed a strong, positive association between VIP levels and clinical activity, both at baseline (r = 0.38, P less than 0.001) and follow-up (r = .41, P less than 0.001). The ability of the VIP immunoassay to gauge clinical activity was also evaluated where VIP concentrations above 30 pg/ml were defined as abnormal. At baseline, sensitivity (specificity) was found to be 81% (55%). The predictive value of a positive (negative) test was 57% (80%). These estimates did not differ at follow-up. Examination of paired plasma samples from intermittently active patients revealed nearly twofold increases (P less than 0.05) in VIP concentration during active periods of disease. The data suggest that plasma VIP levels may be a valuable laboratory parameter in gauging activity in inflammatory bowel disease.