It has long been claimed that a specific histologic diagnosis of mycosis fungoides cannot be made in the premycotic" or "eczematous" (patch) stage of the disease. Indeed, the histologic features of the premycotic lesions were constantly said to be those of "chronic non-specific dermatitis." We studied 46 biopsy specimens of patch lesions from patients in whom mycosis fungoides was unequivocally established by clinical events (i.e., concurrence or later development of typical plaque and/or nodular lesions) and indubitable histologic findings. We divided patch lesions into early nonatrophic patches and late atrophic ones. The early patches are considered to be evolving lesions of mycosis fungoides, whereas late patches represent resolving plaques of the disease. On the basis of this study, we concluded that histologic diagnosis can be made with near certainty in patch lesions of the disease. We found that the critical feature for histologic diagnosis of early and late patch lesions of mycosis fungoides is the presence of an increased number of mononuclear cells distributed singly or in small collections within an epidermis devoid of spongiotic microvesiculation. Other important features are lacunae surrounding intraepidermal mononuclear cells which gives them the appearance of "haloed cells." A sparse infiltrate of mononuclear cells is present around the blood vessels of the superficial, and sometimes the deep, vascular plexus. Atypical mononuclear cells are not necessary for the diagnosis of early patch lesions of mycosis fungoides, but they are found commonly in late patch lesions. Late atrophic patches show a thinned epidermis, loss of the usual configuration between rete ridges and dermal papillae, and coarse collagen throughout a thickened papillary dermis.