Agents recently recognized as causes or potential causes of waterborne outbreaks include pathogenic bacteria (Campylobacter jejuni, Yersinia enterocoliticia), viruses (rotavirus, Norwalk virus and other poorly defined viral agents) and Giardia lamblia, a protozoan agent. Although not a cause of waterborne disease in the classical sense, concern also has been expressed about the relationship of water supplies to the occurrence of legionellosis caused by Legionella pneumophila. New disinfection information on hepatitis A virus (HAV), long established as a cause of waterborne disease, has recently been developed and is included in this report. With the possible exception of Giardia lamblia, waterborne outbreaks caused by these agents seem to occur mainly as a result of deficiencies in treatment, massive contamination of water sources that received inadequate treatment, or contamination after treatment. There is little to indicate that such outbreaks occur in well-operated systems with adequate or conventional treatment.