Inhaled combination therapy composing of long-acting β2-agonist and corticosteroid has been widely applied in the management of asthma, but observed treatment responses vary. The aim of this study was to evaluate the pharmacogenetic effect of the adenylyl cyclase type 9 (ADCY9) gene polymorphism on combination therapy.Eighty-six mild to moderate Korean asthmatics were enrolled in this clinical trial. After the 2-week 'run-in' period, patients received budesonide (an inhaled corticosteroid) and formoterol (long-acting β2-agonist) during the following 12-week active treatment period. Forced expiratory volume in 1 s (FEV(1) ) and maximum mid-expiratory flow (MMEF) levels were measured at all visits as primary outcome. ADCY9 (Ile772Met, 150127 C/T, 150130 C/T, 150397 C/T, 150479 C/T, TTTA (5/4) ) and β2-adrenergic receptor (ADRB2, Arg16Gly) gene polymorphisms were genotyped.Significant associations were observed between the ADCY9 single nucleotide polymorphisms and percent changes in FEV(1) (Ile772Met T/C, P = 0·030) and MMEF (150397 C/T, P = 0·016) after 8 weeks of combination therapy. Haplotype associations were also observed with respect to percent changes in FEV(1) after 8 weeks of therapy (Ht3[TTCC], P = 0·017). Additive therapeutic effect was observed in those with the ADCY9 Ile772Met and ADRB2 Arg16Gly gene polymorphisms in terms of percent change in FEV(1) after 8 and 12 weeks of therapy (P = 0·002 and P = 0·027 respectively).Our results suggest that ADCY9 gene polymorphisms may alone, and in combination with ADRB2 gene polymorphisms, contribute to individual response to combination therapy in mild to moderate asthmatics.