RAST represents the standard technique to titrate serum-specific IgE and was found to have a higher efficiency in the diagnosis of IgE-mediated allergy than other in vitro tests. Pharmacia CAP system (CAP) is a new solid-phase immunoassay, fully automated, used for the titration of specific IgE antibodies. Results are listed in kilounits per liter, equilibrated against the World Health Organization standard for IgE. RAST and CAP were compared in 106 unselected patients (6 to 59 years) characterized by a detailed clinical history and skin prick tests with standardized allergen extracts. IgE to cat, Dermatophagoides pteronyssinus, Alternaria, orchard grass, olive, and Parietaria pollen were tested; 470 tests were run. The specificity, sensitivity, and efficiency of both in vitro tests ranged from 85.5% to 100% except for olive pollen, in which the sensitivity of both in vitro tests was low (68.2% for the new test and 63.6% for RAST). Except for orchard-grass pollen, the sensitivity and specificity of CAP were better than that of RAST. There was a highly significant correlation between both tests (r range, between 0.864 to 0.987). CAP competes favorably with RAST and has the advantage of being automated and eliciting results in kilounits per liter.