General practitioners in 69 practices in England and Wales monitor the spread of epidemic diseases in the community through the Weekly Returns Service (WRS) of the Royal College of General Practitioners, which has existed for over 30 years. Participating general practitioners summarise diagnoses and consultation/episode type (new episodes/ongoing consultations) for a defined population (currently about 570,000) and data are extracted to provide the 'weekly return', which includes age specific weekly incidence of new episodes of selected illnesses. The service has been used extensively to measure the burden of influenza and total acute respiratory illness in the community and the impact of enteric infections. It also provides information about illnesses for which there are no other major data sources--for example, chickenpox, scabies, and (historically) mumps. The entire network is electronically linked. Direct links with microbiological laboratories are being forged in order to integrate clinical and microbiological data in defined populations.