Since the introduction in 1998 of an adolescent pertussis vaccine booster (for persons aged 11-13 years) in France, the incidence of pertussis in adolescents and adults has been unknown. We therefore undertook a study to estimate the incidence of pertussis in these population groups and to evaluate the feasibility of a real-time electronic surveillance system for pertussis in general practices in France. The general practitioners selected for the study were located in Paris and the surrounding area. Polymerase chain reaction (PCR) or measurement of anti-pertussis toxin IgG levels by enzyme-linked immunosorbent assay (ELISA) was used to confirm the infection. Among the 204 patients enrolled in the study, 46 (23%) were diagnosed as having pertussis: 21 were confirmed cases, 24 were clinical cases and one was an epidemiological case. The median age of the 204 patients was 44 years and 134 (66%) were female. The median duration of the patients' cough at enrolment was 24 days. No clinical difference was observed between those with and without a pertussis diagnosis. The incidence of pertussis was estimated to be 145 (95% confidence interval: 121-168) per 100,000 population based on the results from the 10-month study period (calculated for 12 months). Problems in sample collection were identified: pertussis sentinel surveillance cannot be developed without training the staff of medical laboratories who take the biological samples. French health authorities were alerted and training procedures were developed.