This report describes a case of hairy cell leukemia (HCL) with typical hematological and clinical findings. The most striking feature is the electron microscopic pattern of intracytoplasmic inclusions within hairy cells (HCs), which can be identified with the 'tubuloreticular inclusions', very unusual and not yet reported in HCL. The same structures were frequently detected in peripheral mononuclear cells from patients with acquired immune deficiency syndrome (AIDS) and with chronic lymphadenopathy syndrome (LAS), that are caused by the HTLV-III retrovirus. In the same patients elevated serum interferon levels were also found. The close relationship between tubuloreticular inclusions, viral infections and serum interferon levels suggests an etiologic association between a virus infection and outbreak of HCL. The results of our laboratory investigations also support a role for interferon in this disease.