The effects of an inadvertent injection of sodium hypochlorite into the cheek during irrigation of the right maxillary central incisor root canal are reported. The patient suffered from severe pain, edema, and necrosis of subcutaneous tissues and mucosa. Surgical intervention was necessary to contain the destructive process which extended from the upper lip to the right eye. The histopathological examination demonstrated the high cytotoxicity of sodium hypochlorite on vital tissue.