This study involves an investigation of 185 subjects, aged 35--54 years, a representative sample of persons with chronic obstructive lung disease in a local Swedish population. Bronchial asthma was diagnosed in 86 persons, chronic bronchitis in 69 and a combination of bronchial asthma and chronic bronchitis in 30. Reaginic bronchial allergy as a provoking factor was considered as definite when the case history, an intracutaneous test, a bronchial provocation test and a radioallergosorbent test were all positive with regard to one and the same allergen. This was found in 28% of the asthmatics. In 16% of the asthmatics, the diagnosis of reaginic bronchial allergy was not supported by all the four diagnostic criteria used but was considered as probable since the case history or the bronchial provocation test was positive in association with a positive RAST or intracutaneous test with the same allergen. Reaginic (IgE) antibodies in serum to mites (Dermatophagoides pteronyssinus) were found in 20 subjects, 18 of whom also had a positive intracutaneous test for house dust. Four of these 20 subjects had definite and 12 had probable reaginic bronchial allergy. One of the remaining four subjects had chronic bronchitis without asthma. The percentage of definite reaginic bronchial allergy may possibly be increased when reliable bronchial provocations with mite allergen can be performed. Increased IgE levels in serum were found in only eight of the 51 asthmatics with reaginic bronchial allergy and in two of the 65 other asthmatics.