Water treatment plants in the United Kingdom at significant risk of cryptosporidiosis and using conventional filtration methods have been required to install 24-h monitoring systems since April 2000. No major waterborne outbreaks have been described since 2001. Small outbreaks have been associated with water. This paper describes such an outbreak. Data from a local multi-agency surveillance system was used to describe the outbreak, including mapping cases against water supply zones. A case-control study investigated hypotheses raised. All cases were genotype 1. Early cases were in the supply zone of a surface water-treatment plant that had met treatment standards. Later cases included residents in a different supply zone that temporarily received water from the implicated plant. Cases reported more consumption of domestic mains water than controls. Descriptive and analytical epidemiology thus supported drinking water as a source of cryptosporidiosis from a plant meeting regulatory requirements. The evidence for setting drinking-water standards needs review.