We evaluated variations in spirometric indices (i.e., forced vital capacity [FVC], peak expiratory flow [PEF], and forced expiratory volume in 1 sec [FEV,₁]) and static respiratory muscle pressures (i.e., maximum static inspiratory pressure [PI_max] and maximum static expiratory pressure [PE_max]) within a span of 12 hr in 60 healthy elderly subjects, 60 young subjects, and 30 Chronic Obstructive Pulmonary disease (COPD) patients. There were no differences among data of FVC, PEF, FEV₁, Pl_max, and PE_max on three separate occasions within a day in the elderly or the young. The mean coefficient of variation (CV) values of PEF and Pl_max on three occasions were 3.0 ± 0.3% and 4.2 ± 0.4% in the elderly, and 2.4 ± 0.2% and 3.7 ± 0.3% in the young, respectively. No subjects had more than 9% CV on each measurement in the study, suggesting that there is no significant daytime variation in measurement of expiratory flow and respiratory pressures in young and elderly people. However, FVC, PEF FEV₁, and Pl_max values in the morning were smaller than those measured at the other two occasions in COPD patients. The results indicate that COPD affects diurnal variation in pulmonary function, but age alone has little impact on diurnal variation.