Cockroach extracts are known to contain powerful allergens. In some parts of the world the majority of asthmatics have been shown to be allergic to cockroaches. We have evaluated the prevalence of sensitization to the cockroaches Blattella germanica and Trogoderma angustum in a Swiss population with chronic, non-seasonal asthma and rhinitis. The sera of 110 patients were screened for specific IgE by the RAST technique. We found 10 positive RAST (class 1 to 2). 7 of the 110 patients were sensitized to cockroaches (6.3%). 3 patients had positive RASTs for both species of cockroach. 2 patients had no other detectable allergy (= 5% of those who were considered as non-allergic before the study). These results suggest that allergy to cockroaches is uncommon in Switzerland but can sometimes be detected. We conclude that in Switzerland, screening for hypersensitivity to cockroaches should not be included in routine allergic investigations.