Some 1% of the UK population derives their potable water from 140,000 private water supplies (PWSs) regulated by Local Authorities. The overwhelming majority of these are very small domestic supplies serving a single property or a small number of properties. Treatment for such supplies is rudimentary or non-existent and their microbiological quality has been shown to be poor in every published study to date. Private water supplies serving commercial enterprises such as hotels, restaurants, food production premises and factories are more frequently treated and subject to closer regulation in the United Kingdom. As a result, it has been assumed that these larger commercial supplies are less likely to experience elevated faecal indicator and pathogen concentrations at the consumer tap which have been observed at small domestic supplies. This paper reports on intensive monitoring at seven commercial private water supplies (six of which were treated) spread throughout the UK serving hotels, holiday parks and food production enterprises. Daily sampling of ‘potable’ water, both at the consumer tap and using large volume filtration for Giardia and Cryptosporidium spp. was conducted over two six week periods in the spring and autumn of 2000. This allowed the effects of short term episodic peaks in faecal indicator and pathogen concentration to be quantified. All the supplies experienced intermittent pathogen presence and only one, a chlorinated deep borehole supply, fully complied with UK water quality regulations during both periods of sampling. Poor microbiological water quality typically followed periods of heavy rainfall. This suggests that the design and installation of such systems should be undertaken only after the likely range of raw water quality has been characterised, which requires a thorough understanding of the effects of flow and seasonality on raw water quality. There is no reason to suspect that the monitored sites are uncharacteristic of other commercial supplies and the results reinforce public health concerns related to domestic supplies. Furthermore, the pattern of contamination is highly episodic, commonly lasting only a few days. Thus, the relatively infrequent regulatory monitoring of such supplies would be unlikely to identify the poor water quality episodes and does not provide the data necessary for public health protection. Although some statistical relationship was found between faecal indicator organisms and the presence of pathogens, the use of FIOs in assessments of regulatory compliance may not always provide a reliable measure of public health risk, i.e. indicator absence does not preclude pathogen presence. The results of this study suggest that a risk assessment system similar to the WHO ‘Water Safety Planning’ approach might offer a more appropriate regulatory paradigm for private water supplies.