The Colorado Department of Health conducted intensive surveillance for waterborne diseases during the three-year period July 1, 1980-June 30, 1983. Eighteen outbreaks of waterborne illness were investigated. Outbreaks involved from 15 to 1,500 ill persons. Giardia lamblia was confirmed or suspected as the agent in nine outbreaks, rotavirus in one, and no agent could be identified in eight. Seventeen outbreaks occurred on surface-water systems; none of these had adequate chemical pretreatment and filtration. Investigation of water systems exhibiting positive coliform results during the first year detected no outbreaks. Activities important to effective surveillance included educational outreach programs to local health agencies, physicians and the public, and the designation of one individual to whom all water-related public complaints and health department inquiries were directed.