We report a cost benefit analyses (CBA) for water interventions in rural populations of developed country sub-regions. A Bayesian belief network was used to estimate the cost benefit ratio using Monte Carlo simulation. Where possible we used input data from recently published primary research or systematic reviews. Otherwise variables were derived from previous work in the peer-reviewed or grey literature. For these analyses we considered the situation of people with small and very small community supplies that may not be adequately managed. For the three developed country sub-regions Amr-A (America region A), Eur-A (European region A) and Wpr-A (Western Pacific region A), we estimate the costs of acute diarrhoeal illness associated with small community supplies to be U$4671 million (95% CI 1721-9592), the capital costs of intervention to be USD 13703 million (95% CI 6670-20735), additional annual maintenance to be USD 804 million (95%CI 359-1247) and the CB ratio to be 2.78 (95%CI 0.86-6.5). However, we also estimated the cost of post infectious irritable bowel syndrome (IBS) following drinking water-associated acute gastroenteritis to be USD 11896 million (95%CI 3118-22657). When the benefits of reduced IBS are added to the analysis the CB ratio increases to 9.87 (95%CI 3.34-20.49). The most important driver of uncertainty was the estimate of the cost of illness. However, there are very few good estimates of costs in improving management of small rural supplies in the literature. Investments in drinking-water provision in rural settings are highly cost beneficial in the developed world. In the developed world, the CB ratio is substantially positive especially once the impact of IBS is included.