Current natural conditions as well as human activities have been promoting changes and increasing stress in the freshwater and marine environment. One of the consequences is the worldwide occurrence of cyanobacterial (blue-green algae) blooms, which is considered a serious environmental and economic problem (ARAOZ, et al 2005). Risk assessment of cyanotoxins is made more difficult by the lack of scientifically-sound toxicological and epidemiological studies. The available animal data is limited, principally chronic or long term effects. The lack of data is reflected in the fact that a WHO guideline has been agreed only for one group of cyanotoxins (microcystins) (CHORUS and BARTRAM, 2003). Until now, there has been paid little attention to the assessment of risk to drinking water consumers of anatoxin-a, mainly because the suspicion of the rapid excretion of this toxin from the body, no evidence of residual effects and low free-water concentrations in lakes (FALCONER and HUMEPAGE, 2005). Therefore, the aim of this work is to accomplish the risk assessment of anatoxin-a, by approaching three main steps of this process: hazard identification and characterization, exposure assessment, and risk characterization, since the presence of Anabaena sp. blooms capable to produce anatoxin-a are common in Sao Paulo reservoirs, mainly in spring and summer seasons. Additionally, previous data has shown that this toxin can be found in other Brazilian freshwater reservoirs.