An unusual patient with hairy cell leukemia (HCL) who developed marked hepatomegaly due to a large vascular tumor in the liver is reported. The relation of this vascular tumor to the microscopic splenic pseudosinuses and hepatic angiomatous lesions encountered in HCL is discussed. To the best of our knowledge this represents the first case report of the association of HCL with large macroscopic hemangioma of the liver causing hepatomegaly. The patient also developed a large paratracheal mediastinal mass with a recurrent pleural effusion which was shown to contain many typical hairy cells. This rare finding is discussed in relation to the isolated cases of lymphocytic lymphoma who present with clinical and morphological features mimicking HCL. This patient had HCL according to all established criteria with characteristic morphological, cytochemical and ultrastructural features and the pleural effusion and mediastinal mass were most probably part of the HCL neoplasia, despite the fact that biopsy was not performed.