Perfluorinated compounds are highly stable and useful industrial chemicals. Both perfluorooctane sulfonic acid and perfluorooctanoic acid cause immunotoxic effects in animal models at serum concentrations similar to human levels. In children who have undergone routine vaccinations, serum concentrations of these substances are inversely associated with concentrations of antibodies against diphtheria and tetanus. Prevention of such effects will require a decrease of exposure limits by at least 100-fold. Immunotoxicity is not included in routine testing of industrial chemicals but urgently needs attention.