It is generally known that mucositis which often occur on hematopoietic disease patients, shows local necrosis of oral mucosa when the CBC is below the normal range. But sometimes, the lesions are occasionally infiltrate into adjacent tissue. When the pathologic destructive expansion is occurred, differential diagnosis with fungal infection, one of opportunistic infections, is needed. This means treatment and prognosis can be changed according to the diagnosis. So the diagnostic process is more important in this hematopoietic disease patients. In case of fungal infection, the range of tissue damage can expand broadly, and also proper antifungal agent and surgical extirpation should be done. After operation, continuous antifungal therapy and observations are needed. We made a comparative study of following 2 cases of fungal infection appeared on the acute lymphatic leukemia patients to discuss what the proper surgical treatment and medications are, and when the proper surgical intervention time is.