Pulmonary function is predictive of morbidity and mortality. Therefore, in epidemiologic studies, researchers seek to measure pulmonary function with portable spirometers feasible for use in clinics or participants' homes. The purpose of this study was to validate a hand held spirometer, the Welch-Allyn Pneumocheck, against standard pulmonary function tests. The authors used a convenience sample of 66 subjects. All subjects were asked to perform three vital capacity maneuvers with the Welch-Allyn Pneumocheck and then either a water seal spirometer or heated pneumotach in a hospital pulmonary function laboratory. The mean forced vital capacity (FVC) obtained with the Pneumocheck was 3.50 liters (standard deviation (SD) = 1.28) compared with 3.31 liters (SD = 1.24) for the pulmonary function laboratory. The correlation between the two measures was 0.98; the mean difference was 0.19 L (95% confidence interval = 0.12 to 0.26). The mean forced expiratory volume at one second (FEV1) was 2.31 L (SD = 1.07) for the Pneumocheck compared with 2.32 L (SD = 1.06) for the pulmonary function laboratory values. The correlation between the two measures was 0.99; the mean difference was 0.01 L (95% confidence interval = -0.03 to 0.04). Of the 46 subjects originally classified by the Pneumocheck as having FEV1/FVC ratios of less than 0.75, 10 (22%) had ratios greater than or equal to 0.75 after correcting for the FVC difference between methods. Therefore, there was an excellent correlation between the Welch-Allyn Pneumocheck and pulmonary function laboratory values for FVC and FEV1. Small systematic differences in FVC measures can lead to substantial misclassification rates when evaluating FEV1/FVC ratios.