The Valchlor PROVe study is a multi-center, prospective, observational, US-based drug study that longitudinally follows patients with MF-CTCL who are receiving therapy with Valchlor. Patients will be followed prospectively for a maximum of 1 year from the date of signed informed consent (enrollment) until end of study. Continuation in the study is not contingent on continuation of Valchlor.