Pericardial effusion and cardiac tamponade have been described as GVHD manifestations in the post transplant period. Direct evidence of GVHD-related TCR or B-cell receptor clones in patients with pericardial effusion has never been described. Using several methods, including FACS and spectratyping analysis to assess T- and B-cell clonality and to quantify TCR excision circles to assess newly thymus-derived T cells, we were able to show expansion of oligoclonal T-cell populations and the possible presence of early/premature B cells in the pericardial effusion but not in peripheral mononuclear cells. This may explain the presentation of an isolated GVHD manifestation.