LAT is a 36-kDa transmembrane protein that plays an important role in linking engagement of the T-cell antigen receptor (TCR) to the biochemical events of T-cell activation. It has been shown that LAT reacts with human T cells in normal and neoplastic lymphoid tissues, without restriction to any T cell subpopulation. This suggests that the expression of LAT in vivo may be a valuable addition to the panel of immunohistochemical markers used for immunostaining T cells. The expression of LAT has not yet been studied in human pathological skin conditions. We present our experience concerning LAT expression in both neoplastic and inflammatory dermatoses using an immunohistochemical approach on frozen sections from 42 patients. A variable reduction in LAT expression was observed in almost all the inflammatory and neoplastic skin conditions investigated, irrespective of the particular disease. Our study indicates that LAT(-) T cells are more common within the skin T-lymphoid infiltrate than was previously demonstrated in both normal and neoplastic lymphoid tissues. These findings suggest that, using a conventional immunoenzymatic approach on fresh frozen sections, LAT staining is an unreliable marker for the identification of T cells in human pathological skin conditions.