We evaluated the diagnostic value of recombinant Aspergillus fumigatus allergen I/a (rAsp f I/a) for skin testing in cystic fibrosis (CF) patients with allergic bronchopulmonary aspergillosis (ABPA), allergy to A. fumigatus and in CF controls not allergic to A. fumigatus. Skin prick test reactions to rAsp f I/a were 8.5-fold stronger in A.-fumigatus-allergic and 3.3-fold stronger in ABPA patients compared to the CF controls. Three out of 6 well-characterized ABPA patients had positive skin prick test reactions to rAsp f I/a; the same number reacted to crude A. fumigatus extracts. The skin prick test results to rAsp f I/a were significantly correlated with those to crude and commercial preparations. According to these results, rAsp f I/a represents a defined allergen that provides the possibility to perform skin prick tests in CF patients in a highly standardized way. In future, a larger panel of recombinant allergens might also pick up those CF patients with ABPA and allergy to A. fumigatus who did not react to this single allergen.