Plasma venom concentrations were measured by enzyme-linked immunosorbent assay at the time of hospital admission in 56 patients stung by the scorpion Tityus serrulatus. According to clinical and laboratory manifestations patients were classified into two groups: patients with only local pain at the site of the sting (n = 37), and patients with systemic manifestations of envenoming (n = 19). Circulating venom concentrations of these two groups of patients were cross-tabulated and the results showed that patients with systemic manifestations of envenoming had significantly higher plasma venom concentrations than patients with only local pain at the site of the sting.