The authors investigated the relationship between respiratory symptoms in adults and exposure to mite and cat allergens, the role of endotoxins in house dust, the effects of mixtures of several allergens, and interactions between allergen exposure and allergic sensitization. Within a nested case-control study, 405 subjects aged 25-50 yrs from two German cities answered a standardized questionnaire. Allergen-specific immunoglobulin-E was measured. Dust samples were taken from the subjects9 homes to determine exposure to mite (Dermatophagoides pteronyssinus antigen 1 Der p 1) and (D. farinae antigen 1 Der f 1) and cat (cat antigen d1 Fel d 1) allergen and endotoxin content in settled house dust. Exposure to Der f 1 and Der p 1 plus Der f 1 w10 mg?g-1 of mattress dust, respectively, increased the risk of wheeze and breathlessness (odds ratios (OR): 4.04, 95% confidence interval (CI): 1.53-10.64, OR: 2.78, 95% CI: 1.06-7.28). Fel d 1 w8 mg?g-1 was positively associated with cough at night (OR: 2.74, 95% CI: 1.22-6.17), noteworthy also in the nonsensitized subjects. Subjects exposed to elevated concentra- tions of more than one allergen had an up to seven-fold increase in the risk of respiratory symptoms, compared to nonexposed subjects. Sensitized subjects exposed to elevated concentrations of Der f 1o rFel d 1 were found to have the highest risk of asthma attacks and respiratory symptoms. No statistically significant association was found between exposure to endotoxins and respiratory health. Indoor exposure to Dermatophagoides farinae antigen 1 and cat antigen d1 is a risk factor for respiratory symptoms in adults, and for cat antigen d 1 even in nonsensitized subjects. The risk is increased if subjects are exposed to a mixture of allergens or if they are sensitized in addition to high exposure. Eur Respir J 2001; 18: 555-563.