Lichen myxedematosus (LM) is an idiopathic cutaneous mucinosis; its classification dates back to 1953, when Montgomery and Underwood distinguished 4 types of LM: a generalized lichenoid eruption, later called scleromyxedema, a discrete papular form, a localized or generalized lichenoid plaque form, and an urticarial plaque form. In the literature, the terms LM, papular mucinosis, and scleromyxedema have been often used indiscriminately as synonyms, but most reported cases of LM or papular mucinosis without indication of the subtype appear in fact to be cases of scleromyxedema. On the basis of personal experience, the anatomoclinical manifestations of published cases of LM, papular mucinosis, and scleromyxedema are reviewed to distinguish clearly between a generalized form with systemic, even lethal, manifestations and a localized form, which does not run a disabling course. LM includes two clinicopathologic subsets: a generalized papular and sclerodermoid form (also called scleromyxedema) and a localized papular form. Diagnosis of scleromyxedema should fulfill the following criteria: (1) generalized papular and sclerodermoid eruption; (2) mucin deposition, fibroblast proliferation, and fibrosis; (3) monoclonal gammopathy; and (4) the absence of thyroid disease. The criteria for localized LM are as follows: (1) papular or nodular/plaque eruption; (2) mucin deposition with variable fibroblast proliferation; and (3) the absence of both monoclonal gammopathy and thyroid disease. The localized form is subdivided into 5 subtypes: (1) a discrete papular form involving any site; (2) acral persistent papular mucinosis involving only the extensor surface of the hands and wrists; (3) self-healing papular mucinosis, of a juvenile and an adult type; (4) papular mucinosis of infancy, a pediatric variant of the discrete form or of acral persistent papular mucinosis; and (5) nodular form. A third group of atypical or intermediate forms, not meeting the criteria for either scleromyxedema or the localized form, includes cases of (1) scleromyxedema without monoclonal gammopathy, (2) localized forms with monoclonal gammopathy and/or systemic symptoms, (3) localized forms with mixed features of the 5 subtypes, and (4) not well-specified cases. (J Am Acad Dermatol 2001;44:273-81.)