To our knowledge, no one has conducted a multi-center trial evaluating the efficacy of antivenom and cepharanthine (CEP) for the treatment of mamushi (Gloydius blomhoffii) bites. Thus, we conducted a large-scale survey among tertiary care centers in Japan from November 2009 to October 2010 to evaluate the efficacy of antivenom and CEP for the treatment of mamushi bites. We divided the therapeutic interventions received by patients into 4 groups: CEP, antivenom, both CEP and antivenom, and neither CEP nor antivenom. We collected data on age, sex, comorbidities, laboratory measurements, length of hospital stay, and grades of mamushi bites (indication of bite severity ranged from I [mild] to V [severe]). We sent questionnaires to 219 tertiary care centers, of which 114 (52.1%) returned completed questionnaires. Two hundred and thirty-four cases of mamushi bites were reported. Among the severe cases (grades of mamushi bites III, IV, and V), patients administered antivenom had a significantly shorter length of hospital stay than those administered CEP (P = 0.024). In contrast, there was no significant difference in the length of hospital stay between mild cases (grades of mamushi bites I and II) (P = 0.77). Our results show that antivenom is effective in reducing the length of hospital stay in patients with severe mamushi bites.