The purpose of this study was to investigate the effects of house dust mite allergen in bedroom floor dust on respiratory health of children with asthmatic symptoms. Two hundred and twenty eight school children with reported attacks of shortness of breath with wheezing in the past year and/or with doctor-diagnosed asthma, were included in the study. Data on home characteristics, both past and present, were obtained. These included data on allergen avoidance measures because of the child's respiratory health. Dust samples were taken from the child's bedroom floor, and the allergen Der p I of the house dust mite Dermatophagoides pteronyssionus was measured. Health diaries were kept over 4 weeks. Acute respiratory symptoms and medication usage were recorded daily. Peak expiratory flow (PEF) was measured using Mini-Wright peak flow meters three times daily. Levels of Der p I in dust from carpeted floors were significantly higher than in dust from smooth floors. We found a positive relationship of Der p I levels, in floor dust collected from carpeted floors, with PEF-variability and also with the prevalence of wheeze, shortness of breath, and attacks of shortness of breath with wheezing during the observation period. The effects on peak flow variability was larger in children allergic to house dust than in children not allergic to house dust. Peak flow variability was significantly increased at exposure levels well below 10,000 ng Der p I.g-1 dust, which has been suggested to be a "threshold" for increased risk symptoms among sensitized asthmatics.(ABSTRACT TRUNCATED AT 250 WORDS)