The influence of exogenous factors in the home on the circadian variation of airway obstruction has not been fully assessed in children with asthma. The aim of the present study was to investigate the contribution of exogenous stimuli to the degree of peak expiratory flow (PEF) variability during 24 h. Fifty-five children (33 boys and 22 girls; mean age, 9.3 +/- 1.7 yr) with symptoms of asthma, increased bronchial responsiveness, and a solitary allergy to house dust mite (HDM) participated. Their asthma symptoms were well-controlled for at least 4 mo with daily inhaled corticosteroids (ICS) and beta 2-adrenergic drugs if needed. Symptoms, peripheral blood eosinophils, total IgE, and specific IgE to HDM were assessed. Spirometry and PC20 histamine were performed. PEF amplitudes during 24 h (highest minus lowest as a percentage of the day's mean value) were obtained at home during and 6 d after withdrawal of ICS. Dust samples were collected from the total area of the living rooms, bedrooms, mattresses (n = 25), and classrooms to obtain the HDM allergen (HDMA) exposure to Der p I and Der p II. Family smoking habits, presence of pets, and types of floor-covering were recorded on a checklist. Mean PEF amplitude did not increase after withdrawal of ICS, but absolute PEF values were significantly lower (p = 0.05) at midnight and 4:00 A.M. Twenty-six children (47%) were exposed to environmental tobacco smoke (ETS), and 23 (42%) kept pets. Mattresses contained significantly higher amounts of HDMA compared with other locations. PEF amplitude after withdrawal of ICS was significantly higher in children exposed to ETS, a pet, or a high HDMA level in their mattress than in children who were not exposed (ETS: 29.7% [3.9 to 56.6] versus 19.4% [0.0 to 56.6], p < 0.05; pets: 31.4% [9.7 to 52.5] versus 21.9% [0.0 to 56.6], p < 0.05; high HDMA level in the mattress: 35.5% [10.2 to 56.6] versus 21.4% [3.9 to 56.6], p < 0.05). These factors combined with age and PC20 histamine and its interaction with ETS, especially in mild to moderate asthma, explained 48.4% of the variance of the PEF amplitude after withdrawal of ICS. Exogenous stimuli such as exposure to ETS, pets, and high HDMA levels in mattresses contribute to an increased circadian PEF amplitude after withdrawal of ICS and therefore to nocturnal worsening of asthma in HDM-allergic asthmatic children. Moreover, ETS exposure seems to especially worsen PEF variability in children with mild to moderately severe bronchial responsiveness.