A group of 11 female patients (mean age 33.7 +/- 8 years) with a clearly proven primary Raynaud's syndrome of up to five years' duration were subjected to a two-month oral treatment with 3 X 400 mg pentoxifylline per day. The following parameters were studied without and with exposure to cold conditions: hemodynamics (finger photoplethysmography), red cell deformability (filtration test), various clotting variables (prothrombin activity, antithrombin III, plasma fibrinogen, partial thromboplastin time, thrombin time, thrombelastogram), and clinical symptomatology. After treatment 7 of the 11 patients showed a distinct improvement of peripheral blood flow and of symptoms (decrease or removal of asphyxia attacks, pain, color change) under basal conditions, as well as after exposure to cold. Red cell filtration was significantly (p less than 0.05) improved, increasing by 35% under normal conditions and by 30% after exposure to cold. Positive changes were also found in respect to antithrombin III (increase) and plasma fibrinogen (decrease). The thrombelastogram was unchanged. Clinical and instrumental improvements were probably ascribable to better microcirculatory flow due to increased red cell deformability, reduced viscosity, and decreased fibrinogen, all capable of influencing in various degrees the blood flow at the microcirculatory level.