OBJECTIVES--To examine the immunohistological features in the involved skin of patients with psoriatic arthritis (PA) (n = 15), compared with those in involved skin from patients with psoriasis but no arthritis (n = 5), and with a group with normal skin (n = 4), to identify markers for arthritis in psoriasis. METHODS--Skin was obtained from patients by 6 mm punch biopsy and normal skin was provided by the department of plastic surgery. Samples were stained with monoclonal antibodies against T cells (CD3, CD8, CD4, CD45Ro), B cells (CD20), macrophages (mac387), vascular endothelium (FVIII-related antigen) and a Langerhan's cell marker (p155). The number of cells/vessels staining with each monoclonal antibody was calculated and serial sections of skin were examined to estimate the presence of DR+ keratinocytes. RESULTS--There were significantly more CD45Ro T-cells and blood vessels in patients with psoriatic arthritis compared with both psoriasis alone, and with normal controls (p < 0.02). While B-cells were not seen in psoriasis without arthritis or in normal skin, a small but significant number were observed in PA (p < 0.02). Furthermore, while DR+ keratinocytes were present in both psoriatic arthritis and psoriasis skin, there were significantly more DR+ cells in the psoriatic arthritis epidermis compared with psoriasis alone (p < 0.02). CONCLUSIONS--This study suggests that increased numbers of CD45Ro T-cells, greater vascularity, the presence of B-cells, and increased numbers of DR+ epidermal cells are markers for arthritis in patients with psoriasis.