Alum (aluminium sulfate) has been evaluated as a less-hazardous chemical for cyanobacteria that does not cause physiological toxicity and causes little or no release of cyanotoxins. In this study, the effect of alum treatment on toxic Microcystis cells was evaluated on weekly and monthly scales using a microcosm experiment designed to simulate common lakes or reservoirs. Alum treatment caused cell damage and subsequent release of large amounts of microcystin-LR (MC-LR). Moreover, MC-LR and Microcystis cells were degraded by microbes that existed in the lake sediment. This is the first study to report a serious release of microcystin by alum treatment. For this reason, care about release of a large amount of hepatotoxic microcystin should be taken when adding alum for cyanobacterial removal in lakes and reservoirs.