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Background: Patients with mild coronavirus disease 2019 (COVID-19) are usually managed in an outpatient setting. Pulmonary functions in this setting have not been explored. This study aimed to determine abnormal lung functions in COVID-19 patients under a home isolation program. Methods: A prospective study was conducted in asymptomatic or mild COVID-19 patients with normal chest radiographs at two medical centers in Thailand. Spirometry data, including forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), peak expiratory flow (PEF), forced expiratory flow at 25-75% of FVC (FEF25-75), and bronchodilator responsiveness (BDR), were collected. Spirometry was performed after disease resolution at baseline and 3-month follow-up. Abnormal lung functions were classified into airway obstruction, restrictive defect, mixed defect, small airway disease, and BDR. Results: A total of 250 patients (58% female) were included. The mean age was 37.4 ± 15.2 years. Asymptomatic patients accounted for 7.6%. Common symptoms included fever (55.6%) and cough (60.0%). Abnormal lung functions were observed in 28.4% of patients, with a restrictive lung pattern (14.4%), airway obstruction (4.8%), mixed defect (0.4%), small airway disease (8.4%), and BDR (2.8%). Significant changes from baseline were noted in FVC (1.21%), FEV₁/FVC (-1.51%predicted), PEF (0.06%), and FEF_25-75 (-2.76%). Logistic regression analysis indicated that a higher body mass index was associated with a lower risk of abnormal lung function. Conclusions: Ventilatory defects were observed in one-third of patients with mild COVID-19 who did not require hospitalization, mainly presenting as restrictive patterns and small airway disease. Even mild cases may have residual pulmonary impairment, warranting further long-term studies.