The article presents a case of 57-year-old woman with the infiltration of rare small lymphocytic B cell lymphoma in the thyroid gland. Initially, the patient was followed-up due to chronic lymphocytic B-cell leukemia diagnosed on the basis of histopathological examination of cervical lymph node. Eight months later, general symptoms occurred along with lymphocytosis and exacerbation of lesions in lymph nodes, and therefore, chemotherapy was started according to COP regimen. After four chemotherapy cycles, further progression of the disease was observed during chemotherapy. Computed tomography (CT) performed at that time showed generalized lymphadenopathy and the presence of an irregular area in left thyroid lobe. On palpation, the thyroid was asymmetrical, with enlarged left lobe and palpable lymph node packages on the left side of the neck. The levels of thyroid hormones and anti-thyroid antibodies were normal. Ultrasound examination of the thyroid gland showed non-homogeneous hypoechogenic structure of the left lobe and complete focal remodeling. Cytological examination of left-lobe lesion obtained during fine needle aspiration biopsy showed multiple small lymphoid cells, suggestive of small lymphocytic lymphoma. To confirm this diagnosis, flow cytometry of the biopsy material sampled from the left lobe was performed showing B cellimmunophenotype: CD19+/CD20+/CD22 dim/FMC-7, CD23+/CD5+, sCD79b-+, CD38-, CD10-, kappa and lambda-/weak reaction. The results of flow cytometry of the thyroid bioptate and blood were nearly identical, confirming leukemic nature of the infiltration in left thyroid lobe. Cytogenetic findings included the presence of 17p deletion (TP53 gene). The patient received immunochemotherapy with alemtuzumab. The progression of the disease occurred in the sixth week of therapy. The treatment was discontinued after 8 weeks due to worsening of patient's general status. The patient died 15 months after the diagnosis.