How useful is the current Total Coliform Rule (TCR) in assessing water systems' - especially small systems' - vulnerability to a waterborne outbreak? The authors determined that the current TCR is not sufficient for predicting a system's vulnerability to an outbreak from protozoa and viruses. Depending on the frequency of sampling, the current TCR may be adequate for predicting a system's vulnerability to bacterial pathogens. The authors concluded that additional microbial indicators are needed to guard against outbreaks from protozoa and viruses. In particular, greater frequency of sampling is needed for smaller communities. Further, routine monitoring should be supplemented with sanitary surveys and other measures to monitor conditions that may indicate potential contamination. Because of these inadequacies, additional revisions may be needed for the TCR. It is worth noting that provisions within the proposed Ground Water Rule may help prevent outbreaks. Water managers should not be complacent with regard to negative findings obtained during routine coliform monitoring and should continually assess potential sources of contamination, because outbreaks can occur even when the TCR has not been exceeded. Includes 12 references, tables.