On May 6, 1998, the Olmsted County Public Health Department initiated an investigation into an outbreak of diarrheal illness that had occurred among people who swam frequently at a local swimming pool. Interviews of swimmers and microbiological testing of stool samples and swimming pool filter material were conducted to determine the source of the outbreak. Twenty-six of 206 swimmers interviewed had illnesses that met the case definition. The most common symptoms were diarrhea (100 percent), abdominal cramps (81 percent), and nausea (77 percent). The median duration of symptoms was nine days. Four cases of cryptosporidiosis were confirmed by stool analysis. The outbreak was found to be associated with swimming at the pool. Public awareness-including an understanding that recreational water facilities should be avoided during diarrheal illness-and proper pool hygiene are vital in preventing cryptosporidiosis outbreaks. Health care providers also must consider testing specifically for cryptosporidiosis when a patient presents with persistent diarrhea.