IDO is considered a promising therapeutic target for immunological cancer treatment. We found that the antihypertensive agent candesartan cilexetil inhibits IDO. The structural modifications provided a >10-fold more potent inhibitor. Structure–activity relationship and docking studies suggested that candesartan analogues uniquely bind to the entrance of the active site in IDO, and not to the haem region. Analogue synthesis, kinetic analysis and cellular activity are also described herein.