Hairy cell leukemia is a chronic, clonal disease of the hemopoetic stem cell. Although our knowledge on its pathogenesis has increased a lot during the previous years, further details are needed to provide a more personalized, and therefore more successful treatment for patients. New advancements in the growing field of epigenetics may contribute to the finding of novel therapeutic target molecules. Similarly to acute leukemia, hairy cell leukemia also possesses a unique microRNA expression pattern. Decreased levels of tumor suppressor microRNAs lead to the elevation of the expression levels of their oncogenic targets. During our investigations on the level of let-7b and miR-124 in the bone marrow sample of a patient suffering from hairy cell leukemia, decreased expression levels have been found in the case of both microRNAs. These alterations also influence the activity of the MAPK signaling pathway, resulting from the regulation of its members by microRNAs including those mentioned above. Our results support new details about the connection between altered microRNA expression levels and signal transduction pathways in hairy cell leukemia.