In February 1996, an outbreak of illness occurred at a hemodialysis clinic in Caruaru, Pernambuco State-Brazil. At this clinic 116 (89%) of 131 patients experienced visual disturbances, nausea, vomiting, and muscle weakness, following routine haemodialysis treatment. Subsequently, 100 patients developed acute liver failure. As of December 1996, 52 of the deaths could be attributed to a common syndrome now called 'Caruaru Syndrome'. Examination of previous years' phytoplankton counts showed that cyanobacteria were dominant in the water supply reservoir since 1990. Analyses of carbon and other resins from the clinic's water treatment system plus serum and liver tissue of patients led to the identification of two groups of hepatotoxic cyanotoxins: microcystins (cyclic heptapeptides) in all of these samples and cylindrospermopsin (alkaloid hepatotoxic) in the carbon and resins. Comparison of victims symptoms and pathology with animal studies on these two cyanotoxins, leads us to conclude that the major contributing factor to death of the dialysis patients was intravenous exposure to microcystins, specifically microcystin-YR, -LR and -AR. In 2000, a review of the Brazilian regulation for drinking water quality, promoted by Brazilian Health Ministry with collaboration of PAHO, incorporated cyanobacteria and cyanotoxins into this new regulation as parameters that must to be monitored for water quality control.