Plasma samples from 20 patients with acquired cold urticaria were studied. The C1-esterase inhibitor activity was found to be low, but the total C1-esterase inhibitor concentration was normal. Prekallikrein, plasmin-alpha 2-antiplasmin complex, and kallikrein-like activity were also found to be within normal limits. Cold-promoted activation of coagulation factor VII occurred in 45% of the patient plasmas and resulted in a considerable drop in C1-esterase inhibitor activity.