The frequency and class of immediate-type hypersensitivity manifestations were studied in 494 snakebitten and scorpion stung patients who were treated with intravenous injections of antivenom sera. These patients were admitted to HC-FMRPUSP from 1983 to 1988. The effectiveness of a hypersensitivity skin test was also investigated. Eighty two out of 320 patients admitted following snake bites (25.6%) had immediate-type reactions consisting of isolated skin lesions (40%), skin lesions plus respiratory manifestations (19%) and gastrointestinal involvement (17%). Anaphylactic shock occurred in ten patients (12%). Thirteen out of 174 patients admitted following scorpion stings had immediate-type reactions (7.5%). There was also a preponderance of skin reactions. Anaphylactic shock was observed in one patient. The positive predictive value of hypersensitivity skin test was 31.8% and its sensibility was 54.8%. These data show that a hypersensitivity skin test is ineffective in predicting immediate-type hypersensitivity manifestations in patients given snake and scorpion antivenom. Considering these results, this test should be eliminated as a routine procedure when treating victims of poisonous animals. These studies indicate that prior to the administration of antivenom anti-histamine (H1- and H2-antagonists) as well corticosteroids should be given by i.v. route in order to prevent or reduce hypersensitivity reactions. Antivenom sera must always be given under continuous medical surveillance by an intravenous route, without dilution, drop by drop for 15-30 minutes.