Infusion phlebitis is the most common complication of intravenous therapy. Six methods of reducing the incidence of infusion phlebitis including inline final filtration, buffers, heparin, hydrocortisone, heparin-hydrocortisone combinations, and frequent set changes were tested in a two part randomized prospective double-blind study of 266 surgical patients. Patients who received filtered fluids had a significantly decreased incidence of infusion phlebitis as compared with that of controls (P = 0.0000001). Of the other methods tested, only the heparin-hydrocortisone combinations achieved any significant decrease in phlebitis (P less than 0.5). Therefore, inline filtration is a highly effective means of decreasing the incidence of infusion phlebitis and should be considered as a routine part of intravenous therapy.