Coliform-free tap water has previously been thought to be unlikely to cause waterborne disease. However, disease outbreaks (especially those caused by Giardia or Cryptosporidium) and endemic waterborne disease risks have been reported in water systems that have not violated either the 1975 or 1989 maximum contaminant level (MCL) for total coliforms. Focusing on the 1975 rule and using coliform data and investigative reports about outbreaks, the authors compared MCL violations for water systems that did and did not experience an outbreak. Coliforms were usually found in a public system during an outbreak investigation, but in the months before an outbreak, they were detected in only half of the systems and caused an MCL violation in only one fourth of them. The violation rate was not significantly different between community systems that experienced an outbreak and those that did not; usually both met the MCL. To safeguard the public against waterborne disease, microbiological monitoring must be supplemented with periodic sanitary surveys and activities that ensure adequate water quality. Includes 14 references, tables.