Factors that influence the time necessary for complete recovery of FEV1 after inhaling histamine were analysed in forty-five children with asthma. These included the initial bronchial obstruction (baseline FEV1), the provocation dose of histamine producing a 20% fall in FEV1 (PD20) and the fall in FEV1 after the histamine challenge. In addition it was also investigated whether a second challenge carried out after complete recovery of FEV1 would produce a reproducible PD20-histamine value. The time for complete recovery varied widely from 15 to more than 75 min. The time needed for complete recovery of FEV1 after the histamine challenge seems to be mainly determined by the PD20 value. The other factors such as initial bronchial obstruction and the fall in FEV1 after the challenge showed no significant relationship with the recovery time. A second challenge with histamine resulted in a highly reproducible PD20 value. The clinical implication of this study is that other tests can only be performed when FEV1 has returned to 95% of baseline.