The two cases reported in this paper allow the differentiation of pagetoid reticulosis from solitary lesions of mycosis on the basis of clinical, histological ultrastructural, immunohistochemical and enzyme histochemical studies. The first patient presented with a slowly growing asymptomatic scaly plaque of 30 years duration. Histology of the lesion was typical of pagetoid reticulosis. Immunohistochemical studies showed that the abnormal cells present in the cutaneous infiltrate were not T lymphocytes and ultrastructural studies suggested these were of histocytic origin. The second patient presented with a poikilodermatous lesion present for 14 years with histology of plaque stage mycosis fungoides. Enzyme and immunohistochemistry showed the predominant T cell nature of the cutaneous infiltrate and electron microscopy showed numerous mycosis fungoides cells. In contrast to generalized forms of mycosis fungoides, no peripheral blood monocyte chemotactic defect was found in either case. Both lesions responded to radiotherapy.