Background. Giant papillae tongue disorder (GRID) is a newly discovered, long-lasting clinical disorder that may develop in organ-transplanted pediatric recipients. The key feature of this disorder is the unique tongue lesion, which comprises swollen fungiform papillae. The aim of this study was to characterize the immunohistopathology of this novel inflammatory condition. Methods. Six organ transplanted children with GRID were included in the study. Routine histopathology and immunohistochemical stainings for CD3, CD4, CD8, CD25, FOXP3, CD20, CD138, CD68, CD1a, CD15, CD23, and mast cell tryptase were performed. Results. Immunohistochemical analyses of the oral lesions revealed a subepithelial infiltrate that was primarily composed of CD3- and CD4-positive T cells, CD20-expressing B cells, macrophages, and CD138-positive plasma cells. The CD20-positive cells did not display the typical B cell morphology, having in general a more dendritic cell-like appearance. The CD138-expressing plasma cells were distinctly localized as a dense infiltrate beneath the accumulation of T cells and B cells. Increased numbers of CD1a-expressing Langerhans cells were detected both in the epithelium and connective tissue. Because no granulomas were observed and only single lesional eosinophils were detected, GPTD does not resemble a granulomatous or eosinophilic condition. Conclusions. We describe for the first time the immunopathological characteristics of a novel inflammatory disorder of the oral cavity, which may develop after solid organ transplantation in children.