Essential thrombocythemia is a hematological disorder characterized by clonal hemopoiesis in the bone marrow and increased number of circulating platelets. It is usually discovered accidentally at the time of routine blood examinations or can become clinically evident with either thrombotic or hemorrhagic complications. In the present article, we describe the case of a 66-year-old woman with pneumonia due to Pneumocystis carinii, who experienced deep vein thrombosis and pulmonary embolism during hospitalization with a subsequent heparin-induced thrombocytopenia. Bone marrow examination performed after clinical improvement revealed the patient to be affected by essential thrombocythemia.