From December 1997 to April 1998, 1060 laboratory-confirmed cryptosporidiosis cases were reported in New South Wales, Australia. In a case-control study, compared with 200 controls, the 100 cases were younger (mean age 4·2 versus 7·1 years; P < 0·0001), more likely to report swimming at a public pool (59% versus 38%; adjusted OR and 95% CI = 2·7; 1·4–5·1) and swimming in a dam, river or lake (OR = 4·8; 1·1–20·3) but less likely to report drinking bottled water (OR = 0·4; 0·2–0·9). In subgroup analyses, in rural areas illness was associated mainly with contact with another person with diarrhoea, and in urban areas illness was associated with swimming in a public pool. Cryptosporidium oocysts were more commonly detected in pools to which at least two notified cases had swum (P = 0·04). Outbreaks of cryptosporidiosis can be prolonged, involve multiple pools and be difficult to control.