Background and Aim of Study: In patients with asthma, worse pulmonary function is associated with worse clinical outcomes. Appropriate treatment of asthma may improve pulmonary function. In this study, we evaluated the improvement of FEV1 (forced expiratory volume in one second) and the proportion of patients with FEV1 < 60% over 5 years of GINA (Global INitiative for Asthma) implementation. Methods: We included all patients with asthma who had been managed following GINA guidelines at the Asthma and COPD Outpatient Care Unit (ACOCU) of the University Medical Center in Ho Chi Minh City, Vietnam from July 2008 to July 2016. The comparisons of pre-bronchodilator FEV1 or the proportion of patients with FEV1 < 60% between each follow-up visit and the baseline visit were performed by using paired Student t test or McNemar’s test, respectively. Results: Among patients with asthma screened from the hospital’s medical records, 1388 patients were included. The mean age was 40.0 ± 16.7 years old; 64.6% were female; 88.5% were non-smokers. Over 5 years of asthma management, FEV1 (% predicted) significantly improved from 77.5 ± 20.8 at baseline to 86.3 ± 17.4 at year 1, 86.2 ± 17.4 at year 2, 85.6 ± 18.2 at year 3, 85.9 ± 16.9 at year 4, and 85.7 ± 16.7 at year 5 (P < 0.0001 for all comparisons). The proportion of patients with FEV1 < 60% significantly decreased from 18.5% at baseline to 7.4% at year 1 (P < 0.0001), 7.3% at year 2 (P < 0.0001), 9.2% at year 3 (P = 0.0006), 7.3% at year 4 (P = 0.0073), and 6.5% at year 5 (P = 0.0455). Conclusion: Pulmonary function of patients with asthma consistently improved over 5 years of GINA implementation.