Immunoglobulin E antibodies to pertussis toxin (PT-IgE) were demonstrated in 15 of 23 (65%) patients with culture-confirmed pertussis. In 6 individuals there was a low-grade PT-IgE response after 6-9 weeks of disease and in 9 a rapid PT-IgE response, appearing 1-3 weeks after onset of symptoms. The PT-IgE antibody levels in immunized individuals were higher than in the non immunized. Following primary immunization of 23 children with a monovalent whole-cell pertussis vaccine (Burroughs-Wellcome, UK) or with an acellular pertussis vaccine (JNIH-6, Biken, Japan) a late low-grade PT-IgE response was found in 8 (35%). In 7/10 children previously immunized with the JNIH-6, a booster injection 16 months later with the same vaccine resulted in a rapidly appearing PT-IgE antibody response. In contrast, none of 13 children initially immunized with the monovalent whole-cell vaccine and then boostered with either this vaccine or JNIH-6 had detectable PT-IgE antibodies after the booster injection. The study shows that IgE-antibodies to pertussis toxin commonly appear in patients with whooping cough and that the kinetics and the magnitude of the response is influenced by previous exposure to the antigen. A PT-IgE response may also follow pertussis immunization.