Mycosis fungoides and Sézary syndrome (cutaneous T-cell lymphomas [CTCL]) are recognized, especially in the early stages, as being a potentially difficult diagnostic problem when they have to be histologically differentiated from a series of inflammatory disorders (ID). The diagnostic value of immunophenotyping is controversial; almost all studies have been semiquantitative. The prognostic implications of the antigen expression in CTCL are not known. To acquire more insight into these problems, the expression of differentiation-associated and activation/proliferation-associated antigens was examined in cryostat sections from 35 patients with conclusively diagnosed CTCL and 19 patients with ID. Positive cells were calculated as numbers/mm2 tissue section. The results were analysed statistically. Single antigen expressions were not helpful for differentiation between CTCL and ID. High percentages (89%-96%) of patients with CTCL stages I, II, and III and patients with ID could be classified correctly (discriminant analysis) by combined quantitative assessment of the expression of antigens Ki67 and Ki1/CD30 in the dermis and CD25 in the epidermis. In later stages (IVa, IVb) the note of correct classifications was worse because of the decrease in discriminating properties of the respective antigens. The biological role of antigen expressions was analyzed together with the clinical parameters (age, stage of disease). Stage was the parameter with the most significant influence on patient survival. CD1a-positive epidermal cells (Langerhans cells) were the only cell population that was significantly (P = 0.011) associated with survival. Death resulting from CTCL was significantly (P = 0.003) less frequent in patients with epidermal CD1a-positive cell densities higher than 90 cells/mm2 in comparison with patients with lower numbers.(ABSTRACT TRUNCATED AT 250 WORDS)