Objectives To investigate the presence of immunoglobulin-like transcript (ILT)4 and costimulatory proteins (CD40, CD80 and CD86), as well as tumour necrosis factor (TNF)- production in antigen-presenting cells (APCs) from patients with psoriatic arthritis, before and after treatment with the antitumour necrosis factor- therapy, adalimumab. Methods Peripheral blood monocytes from patients with psoriatic arthritis and healthy controls were cultured with CD40 ligand (CD40L) to stimulate differentiation to APCs. Cell-surface phenotype was analysed via fluorescence-activated cell sorting. Results CD40L-stimulation resulted in significantly more ILT4(+) monocytes in cultures from control subjects (n=21) than those from patients (n=20). ILT4-positivity on CD40L-stimulated monocytes was negatively correlated with disease activity in patients. Adalimumab treatment resulted in significant increases from baseline in ILT4-positivity, and in decreases in CD40, CD80 and CD86-positivity in monocytes from patients. Conclusion The effect of adalimumab on monocyte surface phenotype may be due to modification of the inflammatory milieu associated with therapy-induced reduction of disease activity in psoriatic arthritis.