Pityriasis lichenoides et varioliformis acuta and pityriasis lichenoides chronica are idiopathic, papular eruptions that exhibit certain clinicopathologic similarities to each other and to lymphomatoid papulosis. In order to determine if these disorders are also similar immunologically, we studied the immunopathology of five biopsy specimens from three cases of pityriasis lichenoides et varioliformis acuta and three biopsy specimens from three cases of pityriasis lichenoides chronica. We then compared them to our prior immunohistologic study of nine cases of lymphomatoid papulosis. Pityriasis lichenoides et varioliformis acuta and pityriasis lichenoides chronica both exhibited a dermal and epidermal infiltrate of CD4+ and CD8+ T cells expressing activation antigens. These were admixed with numerous macrophages. The lesional epidermis was diffusely human lymphocyte antigen (HLA)-DR+ and contained decreased CD1+ dendritic cells. Endothelial cells were also HLA-DR+. Cells bearing the phenotypes of B cells, follicular dendritic cells, or natural killer/killer cells were essentially absent. Except for the lack of large atypical cells, the results resembled those described previously for lymphomatoid papulosis. These findings indicate that pityriasis lichenoides chronica, pityriasis lichenoides et varioliformis acuta, and lymphomatoid papulosis share several immunohistologic features. Together with certain clinicopathologic similarities, they are consistent with the hypothesis that these three disorders are interrelated.