Serum antibody response to pertussis toxin was measured by enzyme-linked immunosorbent assay in 172 patients with clinical symptoms typical of whooping cough. The diagnosis was verified by culture in 100 patients. Serum antibodies were either not detectable or present only at low levels in sera obtained in the early stage of disease. Significant changes in serum levels of IgG, IgM and/or IgA were demonstrated in 143 patients (83%). The lack of comparable increases in most of the other patients may be due to inappropriate timing of serum collection. Thus, detection of antibodies against pertussis toxin in paired serum samples can be used for serological diagnosis of pertussis. However, the presence of IgM and/or IgA in a single serum sample does not confirm a diagnosis of pertussis, since such antibodies were found in healthy adults as well as in patients two years after the disease. High levels of these antibodies are, however, suggestive of on-going or recent disease.