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.

This clinical registry is being established to collect de-identified information on this rare disease. The registry created by assimilation of de- identified coded patient data will centralize information that can be used to improve the management of the many complications of this disease and its treatment. This study is focused on collection of clinically and biologically meaningful endpoints across multiple institutions; as such, it is not focused on a specific set of hypotheses but will collect data that will facilitate such analyses. The investigators will collect information related to the symptoms and the clinical course of the disease, to the complications from the disease and its treatment, presence of minimal residual disease, frequency of relapse and subsequent management, data on novel molecular markers associated with the prognosis.

In conjunction with the Department of Bioinformatics at The Ohio State University, the investigators have created a system for safeguarding the confidentiality and the identity of all patients who agree to participate in this research registry. Each participating institution will be responsible for de-identification of the data, using software developed by The Ohio State University, Department of Biomedical Informatics, before it is used in the registry. Each institution will confidentially maintain a code for linking this information to an individual patient.

Each participating institution will have direct control over the data contained in the registry that is associated with their respective patient population. As a result of these measures, should a patient wish to withdraw from the registry, the responsible institution will be able to immediately remove all records related to that patient from the registry.

Inclusion:

• Patients with classic hairy cell leukemia
• Patients with the variant of hairy cell leukemia.

Exclusion:

• Children are excluded from the study, since Hairy Cell Leukemia wasn't described in children.

This study will be a multi-institutional, international project that will collate clinical information from patients with Hairy Cell Leukemia from participating institutions in the Hairy Cell Leukemia Foundation. Patients within the United States with HCL, who live in remote areas, without access to a HCLFCenter of Excellence, may voluntarily submit their name and contact information, through the Hairy Cell Leukemia Foundation website.