Complications of intravenous therapy with steel needles and small-bore Teflon catheters were compared in a randomized study of 954 cannula insertions. Cannulas were inserted and cared for by an intravenous team following a standard protocol. There were no cases of cannula-related septicemia and only one case of local infection, a cellulitis in the group in which Teflon catheters were used. There was a low incidence of positive semiquantitative cannula cultures in both treatment groups (steel needles 1.5 percent, Teflon catheters 1.4 percent). The risk of phlebitis was significantly greater with Teflon catheters (18.8 percent with Teflon catheters, 8.8 percent with steel needles, adjusted odds ratio 1.87). Steel needles were significantly associated with infiltration (17.9 percent with Teflon catheters, 40.1 percent with steel needles, adjusted odds ratio 0.39). The over-all rate of complications was significantly greater for the group in which steel needles were used (53.8 versus 64.0 percent, adjusted odds ratio 0.72), principally due to the increased risk of infiltration with steel needles. Analysis of the per day risk of infiltration and phlebitis revealed that these relationships were present for each day the cannulas remained in place. We conclude (1) that steel needles and small-bore Teflon catheters can both be used with low risk of infection and (2) that Teflon catheters more frequently cause phlebitis, whereas steel needles infiltrate more readily.