Extracorporeal photopheresis (ECP) is an important therapeutic option in steroid-refractory chronic graft-versus-host disease (cGVHD). Few biomarkers predicting response exist. We measured serum B-cell activating factor (BAFF) in 46 cGVHD patients receiving ECP before and during treatment course. BAFF level at 1 month of ECP predicted 3- and 6-month skin disease response, with BAFF less than 4 ng/mL associated with significant skin improvement and complete resolution in 11 of 20 patients. High BAFF at 1-month ECP associated with a worsening median 6-month skin score and resolution in 1 of 10 patients. BAFF level at 3 months also predicted the likelihood of maintaining skin disease improvement at 6 months. BAFF level was not correlated directly with extracutaneous cGVHD response, although full cutaneous responders exhibited improved extracutaneous organ response rates compared with skin nonresponders (65% vs 35%). This study suggests that early BAFF measurement during ECP for cGVHD represents a potentially useful biomarker in prediction of treatment outcome.