Phlebitis is the commonest complication of intravenous infusion. It has been suggested that it is initiated by venoconstriction at the infusion site, hence treatment with a vasodilator may reduce its incidence. We carried out a prospective double-blind controlled study of the effect of transdermal glyceryl trinitrate on the survival of peripheral intravenous infusions in 340 patients. Fifty-five per cent (94 out of 170) of the infusions failed in the control group compared with 19 per cent (33 out of 170) in the treatment group (chi 2 = 45, P less than 0.0005). The commonest cause of infusion failure was superficial phlebitis: 47 per cent in the control group and 17 per cent in the treatment group (chi 2 = 46, P less than 0.0005). The estimated median time of infusion survival was 74 h in the control group compared with 127 h in the treatment group (log rank chi 2 = 143, P less than 0.0001). We conclude that infusion phlebitis is a common problem in hospitalized patients and its incidence can be effectively reduced by transdermal glyceryl trinitrate.