Factors that facilitate the asthma diagnosis in children are still being investigated. We attempted to assess the duration of a delay in asthma diagnosis in children, and we examined the previously identified risk factors of its occurrence together with the pulmonary function test results such as spirometry and specific airway resistance (sR(aw)) and sought to determine how the measurement of sR(aw) contributes to asthma diagnosis in children. This study was a part of the Asthma Prevention Study in Poland, which focused on increasing early detection of asthma in a community-based cohort. Data were collected using a questionnaire during regular doctor visits. Childrens' medical documentation from the time period between birth and the visit was analyzed. Participants with the duration of an undiagnosed asthma above the upper quartile range of this variable were defined as having a delayed asthma diagnosis. The independent risk factors of the delayed asthma diagnosis included the absence of atopy, allergic rhinitis symptoms, asthma/allergy in a family, and living in the single-parent families. The whole-body plethysmography performed simultaneously with the spirometry revealed the underestimation of reversibility of bronchial obstruction in 16.4% of participants. This underestimation of reversibility of bronchial obstruction was the only modifiable, independent risk factor of the delayed asthma diagnosis in children. Measurement of sR(aw) contributes significantly to asthma diagnosis in children at risk of the delayed asthma diagnosis.