Fine particulate matter (PM2.5) is an important environmental factor affecting human health. However, most studies on PM2.5 and health have used data from fixed monitoring sites to assess PM2.5 exposure, which may have introduced misleading information on the exposure-response relationship. We aimed to assess the effect of short-term personal PM2.5 exposure on lung function in patients with chronic obstructive pulmonary disease (COPD) and asthma. To achieve this, we conducted a longitudinal panel study among 37 COPD patients and 45 asthma patients from Beijing, China. The COPD group and the asthma group completed 148 and 180 lung function tests, respectively. We found that in COPD patients, for every 10-mu g/m(3) increase in PM2.5 exposure at lag2, the FEV1, FVC and DLco decreased by -0.014 L (95% CI -0.025, -0.003), -0.025 L (95% CI -0.050, -0.003) and -0.089 mmol/min/kPa (95% CI -0.156, -0.023), respectively. There was also a decrease of -0.023 L/s (95% CI -0.042, -0.003) and -0.017 L/s (95% CI -0.032, -0.002) in MMEF at lag3 and lag03, respectively. In the asthma group, every 10-mu g/m(3) increase in PM2.5 exposure led to a reduction of -0.012 L (95% CI -0.023, -0.001), -0.042 L (95% CI -0.081, -0.003) and -0.061 L/s (95% CI -0.116, -0.004) in the FEV1, FVC and PEF at lag3, respectively. Our findings suggest that PM2.5 exposure may primarily affect both airway function and lung diffusion function in COPD patients, and airway function in asthma patients.