Contrast media reactions may be classified as anaphylactoid, vasomotor, severe or life threatening, and fatal. Anaphylactoid reactions mimic immunoglobulin E-mediated hypersensitivity in that signs may consist of urticaria, angioedema, wheezing, dyspnea, hypotension, or shock. These reactions occur in 2% to 8% of all contrast media infusions. Vasomotor reactions occur in 5% to 8% of patients and consist of nausea, vomiting, flushing, and warmth. Severe reactions during which there is a concern for life occur about once per 1000 procedures. Fatalities have occurred in from 1:3000 procedures for intravenous cholangiography to between 1:10,000 to 1:100,000 procedures for intravenous urography. The pathogenesis of contrast media reactions is unknown, and various mechanisms may be associated with different clinical features. Radiocontrast media infusions can cause rises in plasma histamine and complement activation by either classic or alternate pathways or nonsequentially, yet adverse reactions may or may not occur. Abnormalities in the complement system or an increased conversion of prekallikrein to kallikrein has been demonstrated in some patients who have had anaphylactoid reactions. It is unknown if these mechanisms can explain the pathogenesis of anaphylactoid contrast media reactions. When patients who have had definite anaphylactoid reactions require a repeat procedure, the incidence of reactions ranges from 35% to 60% for intravascular infusion. Pretreatment with prednisone and diphenhydramine has been demonstrated to reduce this reaction rate to 9% in 465 procedures. Prednisone-diphenhydramine and ephedrine have further reduced the reaction rate to 3.1% in 192 procedures. These results are statistically significant (X2 = 5.4996, p = 0.019). Emergency equipment should be available should a severe reaction occur.(ABSTRACT TRUNCATED AT 250 WORDS)