We studied circadian variation in FVC, FEV1, PEF, TLC, VC, and RV between 9:00 A.M. and 9:00 P.M. and analyzed how this variation affected estimated longitudinal change. Data from 876 adults were obtained in a longitudinal survey of samples from two Dutch areas. Subjects participated in four surveys held at 3-yr intervals between 1975 and 1985. FVC, FEV1, PEF, and VC increased from 9:00 A.M. until noon and decreased afterwards. TLC was constant over the day, whereas RV decreased from 9:00 A.M. to noon. Average variation in FVC, FEV1 and PEF, expressed as percentages of average level, was 4. 8% (200 ml), 2.8% (86 ml), and 3.1% (250 ml/s), respectively. These results are compatible with circadian changes in airway size. No differences in variability were found between men and women. Significantly larger changes between 9:00 A.M. and noon were found in young adults, smokers, and those with respiratory symptoms than in other subgroups. Adjustment for diurnal variation reduced, albeit slightly, residual standard deviations of estimated longitudinal declines. Average diurnal change was large relative to underlying longitudinal change. Its effect on longitudinal change within an individual can therefore be large depending on age, smoking habits, symptomatology, number of visits, time of measurement, and difference in time between measurements. However, when people are measured at random times during the day for at least three visits, or when measurements are made after 11:00 A.M., effects of diurnal variation in pulmonary function on estimated average longitudinal decline are minimal.