The preparation of recombinant hepatitis B vaccines involves using cellular cultures of Saccharomyces cerevisiae, otherwise known as baker's yeast. Prior to vaccine licensure, clinical trials were performed to address whether residual yeast proteins in the vaccines could induce anaphylaxis, including testing for IgE anti-yeast antibody levels. 1-2% of subjects had anti-yeast IgE antibodies before immunization, but demonstrated no significant rise in IgE after HBV. We searched reports in the Vaccine Adverse Event Reporting System (VAERS) for those that mentioned a history of allergy to yeast and then reviewed the adverse events described in these reports for potential anaphylactic reactions. Probable anaphylaxis was defined as the presence of one or more dermatologic symptoms and one or more respiratory, gastrointestinal, or cardiovascular symptoms with onset within 4 h of Hepatitis B vaccination. Possible anaphylaxis was defined in one of two ways: (1) cases that described dermatologic or respiratory symptoms (but not both) occurring within 4h of vaccination; or (2) cases that described one or more dermatologic and/or respiratory symptoms occurring 4-12 h post vaccination. Among the 107 reports of pre-existing "yeast allergies," 11 reports described probable or possible anaphylaxis after HBV. Four additional cases were described after other vaccines. The majority of vaccinees who met the case definitions and had a history of yeast allergies were female, ages ranged from 10 to 64, and symptom onset ranged from 15 min to 5 h after vaccination. No deaths were reported. The small number of reports to VAERS may be partly due to health care professionals observing current contraindications by not vaccinating yeast sensitive individuals. Nevertheless, yeast associated anaphylaxis after HBV in sensitized patients appears to be a rare event.