Tri-County Health Department investigated an outbreak of cryptosporidiosis linked to a community swimming pool. A cohort study was conducted in 37 persons who were invited to the pool party; 12 (57%) of 21 attendees had primary cryptosporidiosis infection. Risk factors for illness included swimming, getting water in mouth, and swallowing water. The pool met chlorination guidelines and used UV light irradiation, a supplemental disinfection technology that inactivates Cryptosporidium. A follow-up survey of the cohort was completed 7-8 weeks after the pool party; four (25%) of 16 non-attendees had secondary cryptosporidiosis infection. The median duration of illness, including patients with recurring symptoms, was 26 days. Clinical response rate to nitazoxanide, a therapeutic agent, was 67%. This study is unique because it describes a cryptosporidiosis outbreak from a well-maintained community swimming pool using supplemental disinfection. It also reports information on disease burden and treatment response.