Black hairy tongue (BHT) is an unusual condition in adults characterized by marked accumulation of keratin on the dorsum of the tongue, resulting in a hair-like appearance. Herein, we have described 15 patients developing BHT after allogeneic stem cell transplantation (allo-SCT). BHT was generally accompanied by other cutaneous manifestations of cutaneous graft-versus-host disease (GVHD) or a precursor of GVHD. Our experience in this series emphasized that histopathologic evaluation is required for seemingly harmless eruptions like BHT in the posttransplantation period. Given the important prognostic implications of GVHD, physicians should be careful when confronted with BHT.