Bordetella pertussis infection is being increasingly recognized as a cause of prolonged, distressing cough (without whooping symptoms) in children and young adults. Diagnosis of infection in this population is important for treatment and surveillance purposes, and may also prove useful in reducing transmission to unvaccinated babies, for whom disease can be fatal. Serum IgG titres against pertussis toxin (PT) are routinely used as a marker of recent or persisting B. pertussis infection. However, collection of serum from young children is difficult, and compliance amongst these subjects to give samples is low. To circumvent these problems, an IgG-capture ELISA capable of detecting anti-PT IgG in oral fluid was devised. The assay was evaluated by comparison to a serum ELISA, using 187 matched serum and oral fluid samples from children (aged 5-16 years) with a history of prolonged coughing, whose serum anti-PT titre had already been determined (69 seropositive, 118 seronegative). The results showed that, using a cutoff of 70 arbitrary units (AU), the oral fluid assay detected seropositive subjects with a sensitivity of 79.7% [95% confidence interval (CI) 68.3-88.4] and a specificity of 96.6% (95% CI 91.5-99.1). Thus, oral fluid titres of >or=70 AU would possess a positive predictive value of 76.2-93.2% for pertussis amongst children with chronic coughs when used as a surrogate for the serum ELISA (assuming disease prevalence of 12-37%). This oral fluid ELISA will greatly assist in the convenience of B. pertussis disease diagnosis and surveillance.