Fibrin deposition is an important histopathological feature of inflammatory skin lesions and is mediated in part, by procoagulants generated by mononuclear leucocytes (MNL). We examined whether MNL from patients with atopic dermatitis or psoriasis generate enhanced procoagulant activity (PCA). MNL isolated from the peripheral blood of 15 healthy control individuals, 15 patients with atopic dermatitis and 15 patients with psoriasis were incubated for 24 h in the presence or absence of bacterial lipopolysaccharide (LPS). MNL or the cell culture supernatants were then added to recalcified human plasma to determine the clotting time. We found that in both atopic dermatitis and psoriasis MNL cultured in the presence or absence of LPS expressed greatly enhanced PCA (p < 0.01 to < 0.002). Supernatants from MNL cultures from patients with psoriasis, but not those from patients with atopic dermatitis, also generated augmented PCA (p < 0.002). In psoriasis, PCA normalized after successful topical treatment with anthralin. We conclude that enhanced PCA is a characteristic feature of MNL in both atopic dermatitis and psoriasis. In psoriasis the enhanced PCA is directly related to disease activity.