We investigated in well controlled asthmatic children whether it is possible to predict by measuring daytime forced expiratory volume in one second, the decline in nocturnal peak expiratory flow rate values after withdrawal of maintenance medication. Forced expiratory volume in one second and peak expiratory flow rate were measured in the outpatient clinic, on the last day with medication. Peak expiratory flow rates were then measured every four hours on days 4, 5 and 6 without medication. Seventeen children showed an amplitude in circadian peak expiratory flow rate values of more than 20% (group I) and nine children showed an amplitude of 20% or less on the three study days (group II). Mean values +/- SEM were 34.7 +/- 2.1% and 10.5 +/- 1.5%, respectively. Forced expiratory volume in one second values were comparable in both groups. Daytime peak expiratory flow rate values before and after withdrawal, remained on the same level in both groups. In group I peak expiratory flow rate values of 24.00 and 08.00 hours on day 6 were significantly lower (p less than 0.05) than on day 4. The results indicate that history and daytime pulmonary function measurements alone, are insufficient to assess the clinical situation and suggest that a decrease in early morning peak expiratory flow rate value (08.00 hours) is an early sign of deterioration of the disease state, after reduction of medication.