Due to extensive treatment switching in the MAVORIC trial, lack of UK regulatory licence for the comparator, overall survival (OS) with mogamulizumab was compared with patients with previously treated advanced mycosis fungoides/Sézary syndrome (MF/SS) in real-world setting. Data were from the Hospital Episode Statistics database (all patients in NHS secondary care system in 2009-2019). Patients were selected according to trial inclusion criteria, then trial and HES samples were matched on selected variables with significant imbalance. The analysis indicated significant improvement in OS for mogamulizumab treatment compared with UK clinical practice (hazard ratio: 0.36, 95% CI: 0.24, 0.53). Results suggest an OS advantage for patients with advanced MF/SS treated with mogamulizumab in MAVORIC trial compared with UK clinical practice.