The acquired cold urticaria (ACU) syndromes consists of nonfamilial heterogeneous disorders characterized by urticaria, angioedema, and occasionally symptoms of hypotension after cold exposure. In a study of 50 consecutive patients with ACU syndromes, it was observed that 70% experienced cold-induced systemic reactions, most frequently with aquatic activities. Patients with ACU syndromes were categorized by their response to an experimental cold-stimulation time test (CSTT) i.e., minimum time threshold of cold stimulation required to induce a coalescent wheal. One subpopulation of patients with ACU syndromes with positive CSTTs of 3 minutes or less experienced the highest incidence (68%; 13/19) of severe systemic reactions with hypotensive symptoms after natural cold exposure. However, 32% of patients with ACU syndromes (6/19) who experienced cold-induced systemic reactions with hypotension had a negative CSTT or a positive test of greater than 3 minutes. These observations indicate that all patients with ACU with active histories of cold urticaria are at risk to develop systemic reactions to cold and should therefore refrain from participating in aquatic activities. In addition, high-risk patients should receive prophylactic medications (i.e., cyproheptadine or doxepin) that are effective in suppressing this disorder. A diagnostic classification of cold urticaria is presented. This classification permits a more specific definition of the various cold urticaria disorders that comprise the ACU syndromes.