The light-microscopic recognition of patch/plaque stages of mycosis fungoides is difficult, but remains as the “gold standard” for the diagnosis of mycosis fungoides (MF). A review of previous publications concerning the light-microscopic histological criteria for recognition of MF is followed by a summary of our recent histological findings involving 100 patients. The quantitative histological assessment included 13 different parameters for each case of patch/plaque-stage disease. The different criteria included parakeratosis, acanthosis, density of upper dermal mononuclear cell infiltrate, degree of lymphocyte atypia, degree of epidermotropism, Pautrier microabscess formation, band-like pattern, presence of basal vacuolar change, spongiosis, papillary dermal fibrosis, presence of eosinophils, and presence of plasma cells. The most significant histological features of patch/plaque-stage MF are:Broad zones of epidermis in which lymphocytes, which are usually not markedly atypical, are in close apposition to basal and lower-level keratinocytes predominantly as single cells in a somewhat linear configuration on the epidermal side of the dermal-epidermal junction. In general, the surrounding keratinocytes do not display any cytopathic changes, i.e., basal vacuolar degeneration or apoptosis.The papillary dermis contains a variably dense mononuclear cell infiltrate that is usually polymorphous and in which there is papillary dermal fibrosis with plasma cells and eosinophils.Pautrier microabscesses are only present in the minority of cases of MF.Representative “threshold” cases of the earliest recognizable diagnostic cases of MF are illustrated. A pragmatic diagnostic approach to patients presenting with a clinical picture suggesting MF is outlined. The potential usefulness of additional ancillary diagnostic techniques such as immunoperoxidase frozen-section stains and T-cell-receptor gene rearrangements are briefly reviewed.