The number of whooping cough notifications has declined in recent years, as vaccine coverage has recovered from the low levels seen in the 1970s and 1980s. Notification of infectious disease is often incomplete, and this study aimed to estimate the extent to which whooping cough is undernotified. We included all cases of whooping cough occurring in the North West Region between 1 April 1994 and 31 December 1996, identified by notification, hospital admission data, or laboratory reports. By combining the three sources, 1239 cases were identified, 69.6% of which were notified. The notification rate was 29.6% (131/442) for cases admitted to hospital and 45.6% (78/170) for laboratory reported cases. Completeness of notification was estimated to be 35.7% (863/2420; 95% confidence interval 30.3-43.4%) using the capture-recapture method. The study confirms that notification of whooping cough is incomplete and suggests that two thirds of cases are not notified.