The overall objective is to develop a clinical data registry that can be used to facilitate research with the ultimate goal of reducing the morbidity and/or mortality and improving the quality of life of patients diagnosed or living with hairy cell leukemia. With approximately 1,000 new cases of this rare disease identified in the US each year, HCL represents 2% of all cases of leukemia in adults. Considering the rarity of this chronic leukemia, the Hairy Cell Leukemia Foundation (HCLF), in partnership with investigators from its Centers of Excellence, seeks to develop a registry to help researchers identify new trends in outcomes, recognize the most effective treatments, discover previously unknown complications of the disease, and design clinical trials for new therapies.