 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.Initially, this study will be launched at three institutions The Ohio State University in Columbus Ohio, M.D. Anderson Hospital in Houston Texas, and the Royal Marsden Hospital in London in the United Kingdom. Patients 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. 