Subjects with bronchial asthmatic symptoms, forced expiratory volume in 1 second (FEV(1)) and FEV(1)/forced vital capacity [FVC] > 80% could show a positive reversibility test with salbutamol in about 25% of the cases. The aim of this study was to evaluate if a limit functional value for spirometry exists where a reversibility test using salbutamol, over this limit, is not necessary to confirm the diagnosis of asthma. Four hundred patients (mean age 31.12 +/- 10.99) with asthmatic symptoms and normal spirometry (mean FEV(1) 96.06 +/- 11.82%, mean FEV(1)/FVC 98.89 +/- 6.03%) were recruited. The number of subjects with reversible airflow obstruction (RAO) was evaluated using the following different criteria of reversibility: FEV(1) and FVC >or= slant 12%, peak expiratory flow (PEF) >or= slant 15%, FEF(25 -75) >or= slant 35%, and at least one of these (ANY). In subjects with baseline FEV(1) > 100% (150 patients), 26 (17.3%) patients showed a FEV(1) increase > 12% and, among the criteria used, 45 patients (30%) showed ANY. In subjects with baseline FEV(1)/FVC > 100% (204 patients), 36 patients (17.6%) showed a FEV(1) increase > 12% and 53 patients (26.8%) showed ANY. In subjects with baseline FEF(25 -75) > 70% (209 patients), 26 (12.44%) and 49 (23.44%) patients, respectively, showed an increase in FEV(1) > 12%, and ANY. In 56 patients with baseline cut-offs (evaluated together) FEV(1) > 100%, FEV(1)/FVC > 100% and FEF(25-75) > 70%, 10 patients (17.85%) showed a RAO with ANY. FEV(1) > 121% or FEV(1)/FVC > 110.8% or FEF(25 -75) > 110% were cut-off values identified in this study where no RAO subjects were found over these limits. In conclusion, baseline FEV(1) > 100%, FEV(1)/FVC > 100% and FEF(25 -75) > 70% cannot be considered cut-offs where it is not necessary to perform a reversibility test with a bronchodilator over these limits. It is improbable to find a positive reversibility test using salbutamol in patients with asthmatic symptoms and FEV(1) > 121% or FEV(1)/FVC > 110.8% or FEF(25 -75) > 110% to confirm the diagnosis of asthma.