The article presents a clinical case of peculiarities of clinical manifestations, diagnostic and therapeutic approaches of undiagnosed chronic myeloproliferative disease, on the background of which Budd-Chiari syndrome (BCS) developed. The results of clinical course, examination, and treatment of a patient with BCS as a manifestation of the hidden course of primary myelofibrosis with the presence of somatic mutation (V617F) in Janus-tyrosine kinase-2 (JAK2) gene in myeloid cells are presented. Standard clinical and laboratory examinations, and cytomorphologic and histologic examination of bone marrow were used. The diagnosis of BCS was confirmed by ultrasound (US) Doppler examination of the portal system vessels. Symptomatic therapy resulted in insignificant positive results. The analysis of this clinical case showed that the development of BCS was due to a chronic myeloproliferative disease that was not diagnosed before the development of thrombosis. Hepatic vein thrombosis was accompanied by the development of fulminant cytolytic syndrome. Along with symptomatic therapy, patient K., female, 32 years old, underwent transjugular intrahepatic portosystemic shunting 1 month after the first symptoms of BCS appeared, which contributed to a significant clinical effect. Seven years after the installation of 4 transjugular intrahepatic portosystemic shunts, the patient's condition remains satisfactory. The uniqueness of this clinical case lies in the presence of 2 serious diseases at the same time: myeloproliferative pathology (primary myelofibrosis) JAK2-positive variant and BCS. Timely diagnosis of both hematological diseases and their complication in the form of hepatic vein thrombosis with fulminant cytolytic syndrome allowed timely prescription of adequate treatment with a good clinical response.