A panel study was conducted in autumn (116 children) and repeated in spring (66 children) to test the hypothesis that the individual variability of peak expiratory flow rate (PEFR) depends on the environmental exposure to tobacco smoke (ETS). PEFR was measured twice a day (morning: PEFR-M; evening: PEFR-E), using individual meters at homes, in children exposed (ETS+) and not exposed (ETS-) to tobacco smoke at home. In examined groups the individual variability of PEFR-M was--on average--8.0% (ETS+; autumn), 8.1% (ETS+; spring), 10.5% (ETS-; autumn) and 7.7% (ETS-; spring). The individual variability of PEFR-E was 8.0% (ETS+; autumn), 7.9% (ETS+; spring), 9.5% (ETS-; autumn) and 7.4% (ETS-; spring). The results of multivariate analysis of within- and between-subject variability showed the presence of statistically significant within-subject variability only in ETS+ group (PEFR-M in autumn; PEFR-M and PEFR-E in spring). With all the limitations of a panel study design the findings suggest that environmental exposure to tobacco smoke in children affects the degree of within-subject variability of PEFR in children.