In Sri Lanka, the usual treatment for severe local envenomation by the hump-nosed viper (Hypnale hypnale) is with polyspecific snake antivenom. We carried out a prospective, randomized, placebo-controlled, single-blind clinical trial to determine the efficacy and safety of polyspecific snake antivenom in the treatment of severe local envenomation by this snake. Sixty-three patients with signs and symptoms of local envenomation by the humpnosed viper Lanka were randomized to receive either polyspecific snake antivenom or normal saline. The two groups were similar in age, sex, time of presentation to hospital, and degree of envenomation. There was no significant difference between the antivenom and placebo groups in the time taken for complete resolution of the local envenomation (5.52 days versus 4.77 days; P = 0.53, by the Mann-Whitney U test). There was a 44.82% incidence of adverse reactions associated with treatment with antivenom. We conclude that polyspecific snake antivenom is not indicated for severe local envenomation by the hump-nosed viper.