Pulmonary function was evaluated by FEV1, FVC, PEF, MEF50%FVC, MEF25%FVC and MMEF in relation with passive smoking exposure in 143 children aged 6-11 years (mean age = 8.5 +/- 1.6). Passive smoking exposure was evaluated by a questionnaire and by measurement of urinary cotinine levels. Children were divided into different groups: no exposure, low exposure and high exposure. The mean values of each pulmonary function test were compared between the groups and between the different evaluations by Student's test. The evaluation of passive smoking exposure by questionnaire and by urinary cotinine levels separated homogeneous groups of children. The urinary cotinine levels were significantly lower in the non-exposed children than in the other groups (p less than 0.001). Pulmonary function tests, especially MEF50%FVC, MEF25%FVC and MMEF were significantly reduced in exposed subjects (p less than 0.05). These alterations are an index of early involvement of the small airways also in passive smokers. The greater the exposure to passive smoking, the lower the values of the pulmonary function tests.