In 1994, an outbreak of cryptosporidiosis occurred in a rural community in Washington State where water was supplied by two deep unchlorinated wells. Confirmed case-patients had a stool specimen containing Cryptosporidium parvum oocysts. Probable case-patients had diarrhea lasting > or = 5 days. Sixty-two households (68.1% of 91) responded to a survey. Eighty-six cases (15 confirmed, 71 probable) were identified, for an attack rate of 50.9% (86/169 residents). Drinking unboiled well water was associated with being a case-patient (relative risk, 1.84; 95% confidence interval, 0.89-3.82), and a significant dose-response relationship was found between water consumption and illness (P = .004). Water that was presumed to be treated wastewater from a piped irrigation system was found dripping along one well's outer casing, which was extensively rusted. Presumptive Cryptosporidium oocysts were found in well water and in treated wastewater. This investigation demonstrates that even underground water systems are vulnerable to contamination.