We have previously found that most occurrences of anaphylaxis to live virus vaccines are caused by gelatin present in the vaccines as a stabilizer. After we published the evidence for the role of gelatin in anaphylaxis, vaccine manufacturers in Japan began to eliminate gelatin from live virus vaccines. In the present study, we tried to estimate its incidence before the gelatin elimination was started. Physicians and vaccine manufacturers submitted serum samples from children with anaphylaxis to measles, mumps, rubella or varicella vaccine to National Institute of Infectious Diseases (NIID) for 3 years from April 1994 to March 1997. Specific IgE to gelatin was assayed at NIID or two manufacturers by the CAP and ELISA methods. There were 44 children with life-threatening severe anaphylaxis (airway obstruction or anaphylactic shock) during the 3-year period, 41 of whom had anti-gelatin IgE. There were 64 children with mild anaphylaxis (without airway obstruction); 62 had anti-gelatin IgE. There were 100 children with only systemic cutaneous signs; 81 had anti-gelatin IgE. The estimates for the incidence of the severe anaphylaxis in 1994-1996 are: 6.84, 7.31, 4. 36, and 10.3 cases per million doses of gelatin-containing measles, rubella, mumps, and varicella vaccines, respectively.