A number of bacterial, viral and other diseases can be contracted by man through exposure to sewage-polluted bathing-water or beach sand. The increasing use of the sea for recreation has led to major concern regarding health hazards to both local and tourist populations. Epidemiological studies attempting to correlate microbiological water quality with health effects have produced different results, leading to a wide variation in recreational water quality criteria and standards applied, and to considerable controversy regarding their implementation. The Mediterranean provides a good example of a problem region where health risks are accentuated as a result of high utilization of bathing areas coupled with long exposure periods. Coastal pollution by sewage is still a major concern and control measures vary considerably. A number of microbiological/epidemiological studies have been carried out since 1953 in an attempt to define the levels of risk following exposure to different concentrations of bacteria in bathing waters. Most of these have been prospective studies whose design involved subject recruitment on the beach itself, classification of swimmers on the basis of immersion of the head in the water, and follow-up interviews after 7-10 days, together with a system for validation of gastrointestinal symptomatology. Practically all studies showed higher morbidity among bathers as compared to non-bathers, but correlation between specific symptoms and bacterial indicator concentrations varied considerably. On the basis of the 1972-1978 Cabelli Environmental Protection Agency (EPA) study in the United States of America, later developed by EPA into a recommended health-effects criterion for marine recreational waters, one would expect 25-40 gastrointestinal cases per 1,000 persons exposed to seawater containing 100 enterococci per 100 ml. Extrapolation of these figures to annual bathing populations indicates the potential magnitude of the problem. Much more work is needed however before a satisfactory dose-response relationship is obtained, principally because of confounding factors inherent in all studies carried out so far, which still require a solution.