Hairy cell leukemia, also called trycholeukemia, is a rare lymphoproliferative disorder with few circulating tumor cells in the peripheral blood. Hairy cells, characteristic of this disorder, are cells that have hair-like cytoplasmic projections. HCL has a chronic course. It is slowly progressive and has a good response to the standard treatment with purine analogues. Usually it affects the spleen, and lymph nodes are spared. HCL is associated with the BRAF gene mutation, more precisely with the most common, BRAF V600E mutation. That mutation leads to the constitutive activity of the RAS-RAF-MEK-ERK signal pathway and cell proliferation. BRAF mutations are related to many malignomas, but most commonly to melanoma. The discovery of the association between the HCL and the BRAF mutation lead to new possibilites in the treatment of the disorder, such as BRAF inhibitors, vemurafenib, and dabrafenib, as well as the possibility to follow up the disease with the RT-PCR method. In our study, 15 patients diagnosed with HCL were analyzed in the Clinical Hospital Dubrava in Zagreb. We used peripheral blood samples and samples of bone marrow embedded in paraffin. In this study we tried to detect the BRAF V600E mutation in patients diagnosed with HCL, and we also evaluated the allele-specific RT-PCR in the follow up of the disease and in the detection of the minimal residual disease. We concluded that the RT-PCR method is sensitive enough to detect the mutation in the active disease and in the relapse, but is less sensitive in the remission and in the minimal residual disease. A high percentage of lymphocytes in the peripheral blood has proved to be a good predictor of the BRAF V600E mutation.