Cyanobacteria (blue-green algae) commonly occur in fresh and brackish water where they produce blooms under certain environmental and climatic conditions. Since some species produce neurotoxins, hepatotoxins, cytotoxins, and endotoxins, blooms can be hazardous for animal and human health. Several cases of human cyanobacterial poisoning have been documented, but accurate assessment of the risk is difficult for lack of knowledge concerning exposure levels and the incidence of this kind of poisoning. Most human cases have been reported after oral consumption of contaminated drinking water or swimming in recreation waters where blooms have occurred. Further study is needed to evaluate and manage this risk, especially in regions dependent on surface water for drinking and recreational water areas. This is especially true in tropical and intertropical areas where climatic conditions promote occurrence of cyanobacteria blooms and nothing is known of the impact on public health.