The validity of Phadiatop as a tool in the mass screening for inhalant allergies was investigated. Seventy-nine out of 600 recruits (13.2%) were classified as allergic to inhalant allergens (68 oculorhinitis; 11 asthma) on the basis of positive history, confirmed by skin testing and/or RAST for the seven most common aero-allergens in Italy. Another 74 subjects had positive RAST and/or skin tests, yet had never experienced allergic symptoms. Phadiatop was positive in 145/600: in 78 out of 79 allergics and in 67 RAST-positive non-allergics. Correlation between Phadiatop and RAST was statistically significant, though higher for grass pollens (r = 0.85, P less than 0.001) than for Dermatophagoides pteronyssinus (r = 0.68, P less than 0.01). PRIST (cut-off 220 IU/ml) identified 36/79 allergics (45.6% sensitivity, whereas that of Phadiatop was 98.7%). As the high percentage of cases in the unselected population had specific serum IgE against inhalant allergens and no allergic symptomatology, the Phadiatop-positive predictive value falls to 53.7%, thus creating the need for Phadiatop-positive subjects to undergo further investigation for an appropriate diagnosis. Given this limitation, Phadiatop appears to be an important step forward in the field of mass-screening programmes for inhalant allergies.