In October 2007 an increase in laboratory-confirmed cryptosporidiosis cases in Staffordshire, England prompted an outbreak investigation. Case ascertainment included interviewing suspected cases and contacts and obtaining faecal specimens from those with diarrhoea for laboratory identification. Over a three-month period we identified 57 cases of cryptosporidiosis (39 confirmed) distributed across 36 households. The majority of cases (69%) were younger than 20 years. The most plausible exposure was multiple swimming episodes (56% of cases) in 13 local public swimming pools. One large swimming pool was most frequently visited by swimmers and considered a significant contributor to transmission because of substandard filtration and maintenance systems. Control measures focused on inspecting and improving operating standards at swimming pools, hygiene information to swimmers, and early detection and exclusion of cases. The rapid case investigation described in this paper provided adequate information for the early detection and control of a typical seasonal swimming pool related cryptosporidiosis outbreak. Ensuring adequate filtration standards at public swimming pools particularly before the high use periods of late summer and autumn remains a priority.