Factor VIII complex and its related activities (Coagulant, Antigen and Ristocetin Cofactor) have been investigated in 23 patients with Progressive Systemic Sclerosis (PSS) divided into two groups: acrosclerosis and diffuse sclerosis. All Factor VIII-related activities were higher in PSS patients than in normal subjects. No difference in F. VIII-related Antigen (F. VIIIR:Ag), F. VIII-related Ristocetin Cofactor (F. VIIIR:Co) and F. VIII Coagulant activity (F. VIII:C) was found comparing the patient groups. F. VIII:C was increased significantly less than F. VIIIR:Ag and F. VIIIR:Co in both patient groups. Some hypotheses about the pathogenesis of this increase are discussed.