The oral cavity is the place where the first symptoms of systemic diseases may appear. Leukemia is the malignancy of the hematopoietic system in which abnormal leukocytes are produced in the bone marrow and these cells spread to the peripheral blood. It is classified clinically on the basis of the duration and nature of the disease (acute or chronic), the type of cell involved (myeloid, lymphoid, or monocytes), and a rise in the number of abnormal cells in the blood. The study aimed to assess and compare the oral hygiene and periodontium status based on the indices in leukemic patients before and after one cycle of chemotherapy and whether the therapy had an impact on the change of these parameters. Dental indices used in clinical diagnostics were calculated: API (approximal plaque index), SBI (sulcus bleeding index), and CPI (community periodontal index). The research project was conducted at the Clinic of Hematooncology and Bone Marrow Transplantation of the Independent Public Clinical Hospital No. 1 in Lublin. The target population consisted of 102 adults with leukemia who were over 18 years of age. The time since diagnosis of the disease ranged from 1 to 10 years. The data were evaluated in the Statistica 12 software with the respective tests. In the majority of patients, both before and after chemotherapy, improper oral hygiene and severe generalized periodontitis were confirmed. The cycle of chemotherapy that was used did not correlate with the change of patients’ oral hygiene and periodontium state. Unsatisfactory oral hygiene and periodontal health has to be addressed with urgent dental treatment to avoid systemic complications in leukemic patients.