To study whether nocturnal bronchial obstruction changes during the year, we assessed the circadian FEV1 variation during four consecutive seasons in 20 children (12 boys; aged 9-12 years) with episodic asthma who were outgrowing their asthma. FEV1 was determined every 4 h between 10:00 and 10:00 during two consecutive days. The last six FEV1 values were submitted to cosinor and coefficient of variation (CV) analyses. The seasonal means (SD) in the group 24 h percent predicted FEV1 was 85.5 (11.4), 81.2 (10.6), 86.0 (11.6), and 82.2 (14.0)% during spring, summer, autumn, and winter, respectively. The difference between the summer and autumn FEV1 values was statistically significant (p < 0.05). The mean (SD) of the circadian amplitude values was 4.1 (4.3), 6.0 (3.8), 4.9 (3.4), and 7.2 (4.1)% during spring, summer, autumn, and winter, respectively. The difference in amplitude between the spring and winter and between the autumn and winter values was statistically significant (p < 0.05). CV values of 48 of the 80 (60%) circadian FEV1 time series exceeded the average CV of 5% observed in nonasthmatic children studied in our laboratory. There was an unequal distribution during the year in elevated CV values; 6, 17, 10, and 15 of the high CV values occurred, respectively, in the spring, summer, autumn, and winter. These results suggest that nocturnal bronchial obstruction may change seasonally in terms of severity and amplitude in children who have nearly outgrown their asthma.