The effects of Petasites hybridus (butterbur) as add-on therapy to inhaled corticosteroids in patients with atopic asthma is currently unknown. Scottish investigators evaluated the effects of P. hybridus, given as add-on therapy to asthmatic patients maintained on inhaled corticosteroids, assessing adenosine monophosphate (AMP) broncho-provocation (primary outcome variable) along with other surrogate inflammatory markers such as exhaled nitric oxide, serum eosinophil cationic protein and peripheral blood eosinophil count. A total of 16 atopic asthmatic patients with mean (standard error of mean) forced expiratory volume in 1 s (FEV1) of 78 (4)% predicted, maintained on their constant dose of inhaled corticosteroids throughout the study, received twice daily for 1 week either P. hybridus, 25 mg, or placebo, in a double-blind, crossover fashion, with a 1-week washout period prior to each randomised treatment. Measurements were made at baselines prior to each randomised treatment and following the randomised treatment period. Baseline values for the primary and secondary outcomes were not significantly different prior to P. hybridus and placebo. AMP provocative concentration causing a 20% reduction from baseline FEV1 (PC20) as doubling dilution change from baseline, significantly improved with P. hybridus, 0.6 (0.2), compared with PL, −0.1 (0.3); a 0.7 doubling dilution difference. Exhaled nitric oxide as change from baseline was significantly reduced with P. hybridus, −1.2 (0.8) parts per billion (ppb), compared with placebo, 0.5 (0.4) ppb. Both serum eosinophil cationic protein and peripheral blood eosinophil count as changed from baseline were also significantly suppressed with P. hybridus.