Unusual clinical manifestations, rare sites of involvement, and associations with other disorders and malignancies occurring in patients with hairy cell leukemia (HCL) are uncommon events encountered in a relatively rare disease. The exact prevalence of these associations is difficult to determine accurately in HCL as they are often anecdotal case reports and not always detailed in all larger series of patients. This short review deals with the unusual clinical manifestations and rare sites of involvement of the disease and lists some of the disorders associated with HCL, based on what has been reported in the literature as well as from personal experience. No attempts are made here to establish the true prevalence of these phenomena and only selected references are included. Some details of the coexistence of HCL with other neoplasias, hematological disorders, and 'paraneoplastic' autoimmune disorders are provided, while opportunistic infections in HCL, particularly atypical mycobacterial disease, are briefly discussed. For the sake of brevity many of the details are provided in tabular form.