An inventory of standing waters (freshwater lakes and lochs) was derived from Ordnance Survey digital map data at a scale of 1:50 000 and represents the most comprehensive survey of its kind for Great Britain. The inventory includes 43 738 water bodies in England, Scotland, Wales and the Isle of Man and contains basic physical data such as location, surface area, perimeter and altitude. Catchment areas were computed for water bodies with a surface area larger than 1 ha from a digital terrain model (DTM) using customised routines in a geographical information system (GIS). The resulting polygons were then used to derive catchment-related information from a variety of national datasets including population density, livestock density, land cover, solid and drift geology, meteorological data, freshwater sensitivity status, acid deposition and conservation status. Using data derived from the inventory a risk-based prioritisation protocol was developed to identify standing waters at risk of harm from acidification and eutrophication. This information is required by the Environment Agency, Scottish Environmental Protection Agency and the U.K. statutory conservation bodies to co-ordinate actions and monitor change under international, European and national legislation.