The hemorrhagic syndrome caused by contact with Lonomia obliqua caterpillars has reached epidemic proportions in southern Brazil. This study aimed at assessing the efficacy of two different dosages of an antilonomic serum (SALon) in the treatment of patients who had accidental contact with L. obliqua caterpillars. A randomized, prospective controlled trial was conducted at São Vicente de Paulo Hospital, a tertiary hospital in Passo Fundo, Brazil. From January 2000 to April 2002, 44 patients with grade I or II hemorrhagic syndrome were randomly assigned to either group A: 22 patients treated with 10.5 mg total dose (three vials of SALon) or group B: 22 patients treated with 17.5 mg total dose (five vials of SALon). Treatment efficacy was assessed according to time necessary for blood coagulation to go back to normal, incidence of adverse reactions, and hospitalization time. There were no differences in clinical picture and laboratory findings between the two groups. Mean time for coagulation to go back to normal was 15.3±6.6 and 19.1±8 h in groups A and B, respectively. Adverse reactions and hospitalization time were similar in both groups. SALon was effective and well tolerated. This study demonstrated that three vials are as effective as five vials of SALon for the treatment of envenoming by Lonomia caterpillars.