Of 34 patients with chronic inflammatory bowel disease (CIBD), 4 developed well-documented episodes of deep-venous thrombosis. All 4 patients had active disease at the time of thrombosis. This group was studied to determine if the tendency to deep-venous thrombosis in patients with CIBD was associated with reduced antithrombin activity by measuring the concentration of three thrombin inhibitors, antithrombin III (AT III), α2-macroglobulin (α2M) and α1-antitrypsin. 2 patients had low AT III levels and 10 had low α2M levels. 2 patients who developed deep-venous thrombosis had significantly low levels of both AT III and α2M. It is suggested that in patients with diseases predisposing to thrombosis and associated with low AT III levels, the measurement of α2M in addition to AT III may predict those particularly at risk.