An outbreak in the autumn of 2005 resulted in 218 confirmed cases of Cryptosporidium hominis. The attack rate (relative risk 4.1, 95%CI 2.8–9.1) was significantly higher in the population supplied by Cwellyn Water Treatment Works (WTW). A case–control study demonstrated a statistically significant association (odds ratio 6.1, 95% CI 1.8–23.8) between drinking unboiled tap water and C. hominis infection. The association remained significant in a logistic regression analysis, with an adjusted odds ratio of 1.30 (95 CI 1.05–1.61) per glass of unboiled tap water consumed per day. This evidence together with environmental and associated microbiological investigations, and the absence of effective treatment to remove Cryptosporidium oocysts at the WTW, led to the conclusion that the outbreak was waterborne. Oocyst counts in final treated water at the WTW and at different points in the distribution system were consistently very low, maximum count in continuous monitoring 0.08 oocysts per 10 litres. Data from continuous monitoring and the epidemic curve is consistent with the hypothesis that low numbers of oocysts of C hominis were present in treated water continuously during the outbreak and these were of sufficient infectivity to cause illness. All surface water derived water supplies present a potential risk to human health and appropriate control measures should be in place to minimise these risks.