In a prospective study of 100 adults, we determined the frequency of hypersensitivity to allergens most often diagnosed in patients suffering from rhinitis and bronchial asthma. Skin-prick tests and RAST for 15 allergens were performed in all patients. The most frequent allergen detected by skin-prick test was the extract "total house dust" (50%) but the corresponding RAST was rarely positive (16%); then came grass pollens (46%), house dust mites (D. pteronyssinus, 38%) and animal danders (cat 33%). On the other hand, a positive skin test with the two moulds chosen was rare (Alternaria tenuis 5%, Cladosporium herbarum 4%). For all the allergens investigated, the correlation between positive skin test and positive RAST was proportional to the intensity of the skin reaction (94% for the skin tests +, 40% for the skin tests ++). 35% of the patients with at least a positive skin test and a RAST for one of the allergens tested had in fact a total IgE concentration not suggestive of atopy (less than 100 kU/1). The results indicate that, with the exception of the 2 moulds chosen, the frequency of allergens diagnosed in Switzerland is very similar to what has been found in other countries with different climates.