BACKGROUND Obesity is the most common metabolic disease in the world. However, the relationship between obesity and lung function is not fully understood. Although several longitudinal studies have shown that increases in body weight can lead to reductions in pulmonary function, whether this is the case with the Japanese population and whether high body mass index (BMI) status alone represents an appropriate predictor of obstructive lung dysfunction remains unclear. The purpose of present study was to estimate the effect of BMI on lung function measured by spirometry of Japanese patients in general clinics. We measured BMI and performed spirometry on screening patients who had consulted general clinics. METHODS Subjects comprised 1231 patients ≥ 40 years of age (mean age (65.0 ± 12.0) years, 525 men, 706 women) who had consulted clinics in Nagasaki Prefecture, Japan, for non-respiratory disease. BMI was calculated and lung function was measured by spirometry. RESULTS BMI was found to be positively correlated with forced expiratory volume in 1 second (FEV(1))/forced vital capacity (FVC) in men and with maximum mid-expiratory flow (MMF) in all subjects. Following adjustment for relevant factors, a significant positive correlation between BMI and FEV(1)/FVC was identified for all subjects. Comparison between subjects with normal BMI (18.5 - 25.0) and higher BMI (25.1 - 30.0) also demonstrated that FEV(1)/FVC and percentage of predicted maximum mid-expiratory flow (%MMF) were significantly higher in the latter subjects. CONCLUSIONS In a population without marked respiratory disease, higher BMI subjects showed less obstructive pulmonary dysfunction compared to normal BMI subjects. High BMI status alone may be inappropriate as a predictor of obstructive lung dysfunction, particularly in populations with a low prevalence of obesity.