In a study on acute health effects of air pollution in the Netherlands, pulmonary function has been measured repeatedly in children aged 7-11 yrs. In study periods of approximately 3 months, 6-10 tests have been made in a group of 1,621 children. The spirometric data have been examined for the presence of trends of pulmonary function with day of study, independent of air pollution. Peak expiratory flow (PEF) increased more than expected from normal lung growth, whereas for maximal mid-expiratory flow (MMEF) a decrease with time was observed. For forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) a smaller than expected increase with time was observed. The observed deviation from the expected pattern was larger for the children with a one week interval between successive tests than for the children with a three week interval. For FVC and FEV1 a non-linear relationship with time was observed, for PEF and MMEF this relationship was approximately linear. The particular changes of spirometric variables with time need to be taken into account when repeated lung function tests are performed to investigate acute effects of air pollution exposure.