A retrospective review of 740 charts of patients with a history of adverse reaction to tetanus toxoid immunization was undertaken. The most common reactions, by history, were local edema and tenderness (33%), fever (15%), and anaphylactoid response (33%). Three patients who had a vesicular eruption at the immunization site were found to have delayed hypersensitivity to mercury. Thirty percent of the patients had received tetanus toxoid within one year and 55% within five years of evaluation. Reactive responses to immediate skin tests were exceedingly rare (less than 1%). None of the challenge patients suffered an adverse reaction.