Mycosis fungoides is a systemic lymphoma that nearly always manifests itself first in the skin as erythematous macules and then patches. Most patients have only patches for a lifetime. Some patients develop subtle plaques in addition to patches. Others, however, in time, display formidable plaques and those lesions may be joined later by nodules and tumors, the latter sometimes becoming ulcerated. Almost always, patients with nodules and tumors of mycosis fungoides have patches and plaques, too. Although plaques of mycosis fungoides may involute in the absence of therapy (the end stage of that regression being an atrophic patch accompanied by hyper- and hypopigmentation and telangiectasis, so-called poikiloderma vasculare atrophicans), that is not the case for nodules and tumors or for patches; they tend to persist and, being manifestations of a malignant neoplastic process (a lymphoma) that they are, grow, albeit slowly as a rule. Ellipsoid patches of mycosis fungoides are designated "digitate dermatosis" and larger patches devoid of specific geometric outline are called "parapsoriasis en plaques." Digitate dermatosis (and a drop-sized variant of it termed guttate dermatosis by us) is referred to as "small-plaque parapsoriasis" and all the other expressions of mycosis fungoides that take the form of flat or very slightly elevated lesions are known as "large-plaque parapsoriasis," to wit, xanthoerythroderma perstans (a patch with a yellow cast), parakeratosis variegata (scaly macules and papules that have become confluent to create a netlike design), alopecia mucinosa (a patch or subtle plaque punctuated by keratotic elevation that are equidistant from one another), and poikiloderma vasculare atrophicans. Sometimes, one or a few plaques are situated in the vicinity of an ankle, that manifestation of mycosis fungoides being designated Woringer-Kolopp disease or pagetoid reticulosis. Broad patches in the axillae and in the groin may become plaques and eventually huge pendulous folds dubbed confusingly granulomatous slack skin. Erythematous macules may become patches that, in turn, become confluent in universal fashion, the result being the erythroderma named eponymically for Sézary.

Flat and elevated lesions of mycosis fungoides, irrespective of the particular appearance of them clinically, have in common specific findings histopathologically, namely, a superficial perivascular infiltrate of lymphocytes, lymphocytes aligned as solitary units in foci of the epidermal basal layer, lymphocytes disposed as solitary units in the spinous zone in conjunction with scant spongiosis, and elongated mounds of parakeratosis that may contain a tad of serum. Sometimes lymphocytes in the epidermis are larger than those in the dermis. Wiry bundles of collagen in haphazard array often accompany patchy lichenoid infiltrates of lymphocytes.