In a phase I trial of anti-ICOS antibody for relapsed/refractory cutaneous T-cell lymphoma, peripheral T-Cell lymphoma, and angioimmunoblastic T-Cell lymphoma, anti-ICOS antibody MEDI-570 was safe, well tolerated, and showed promising clinical activity in poor-risk refractory angioimmunoblastic T-Cell lymphoma. Here, ICOS is linked to primary cutaneous T-cell non-Hodgkin lymphoma.