The aim of this study was to detect the best test lung function to identify abnormalities in asthmatic children while asymptomatic. We studied 200 asthmatic children. Patients were evaluated by questionnaire to evaluate the presence of symptoms and drug consumption in the previous 3 months. Allergological evaluation by skin-prick tests and seric-specific IgE determination for relevant local inhalant allergens was made. Lung function was studied evaluating expiratory flows and measuring lung volumes by nitrogen washout technique. One hundred twenty-seven of 200 (63%) children showed functional abnormalities: 21 (10.5%) children had flow reductions, 56 (28%) children had volume modifications, and 50 (25%) children had either flow and lung volume alterations, respectively. We observed increased total lung capacity (TLC), residual volume (RV), and ratio RV/TLC values more often in patients with mild and moderate persistent asthma than in patients with intermittent asthma. No significant difference was detected considering expiratory flows. Measurements of lung volumes are an important tool to evaluate "air trapping" in asthmatic patients and this could be related to disease severity. According to the literature, our data suggest that an increase of RV, functional residual capacity RV/TLC could be the only functional airway dysfunction present in asthmatic children during asymptomatic period.