Plasmacytoid monocytes (PCMs) are mononuclear cells of monocyte/macrophage origin with peculiar plasmacytoid features on electron microscopy. This cell type is largely unknown in dermatopathology, probably due to standard fixation procedures. The authors provide a detailed histopathologic, immunohistochemical, and electron-microscopic study of PCMs in Jessner's lymphocytic infiltration of the skin (LIS). PCMs occurred in 58% of all biopsy specimens from 75 patients with LIS in single or multiple clusters within the dermal infiltrates. Recognition of this cell type was important for differential diagnostic reasons because PCMs did not occur in polymorphic light eruption. In discoid lupus erythematosus, PCMs were present in only 7.3% of 110 biopsy specimens. PCMs are probably involved in the activation, proliferation, and differentiation of T-lymphocytes. Their origin and function are discussed and a special role in the homing of lymphocytes into the skin is suggested.