Obesity is associated with pulmonary function disturbances. We hypothesized that lung function decreases with increasing duration of obesity. We evaluated pulmonary function tests (PFTs) in 188 nonsmoking subjects with primary obesity (aged 8-76 years; 36% with systemic hypertension). Duration of obesity was assessed by questionnaire in adults, and by height and weight growth patterns in children. Asthma and/or other allergic diseases were investigated by standardized questionnaires. BMI and BMI-standard deviation scores (SDS) were 38.7 and 2.4 kg/m(2), respectively. Forty-six percent of patients were atopic. Among subjects with ever asthma (33%), 20 had current asthma (11% of the total). Forced vital capacity (FVC), forced expiratory volume in 1 s, total lung capacity (TLC), and functional residual capacity (FRC) were 103, 104, 95, and 76% predicted, respectively. Mean duration of obesity was 8.3 years. Compared with subjects who had been obese for ≤5 years, patients who had been obese for >15 years had significantly lower values on PFTs (P < 0.05). In subjects with systemic hypertension, PFTs were lower than in patients without hypertension (P < 0.01). Duration of obesity was significantly related to all PFTs (P ≤ 0.001). In a multiple regression analysis where duration and severity of obesity, hypertension, atopy, asthma, and family history of atopic diseases were independent variables, duration of obesity was a predictor of lower PFTs (P < 0.01). Of the remaining variables, only hypertension contributed to lower lung volumes. In obese individuals, lung function was significantly lower in subjects with greater years of obesity. Fat loss programs should be encouraged to prevent late pulmonary function impairment.