Asthma is an inflammatory disease and interleukin 12 (IL-12) may play a regulatory role in allergen-induced inflammation. The aim of this study was to investigate the association of polymorphisms in IL-12A/IL-12B with asthma. The asthma group included 198 adult patients and the control group included 453 individuals without asthma that were frequency-matched by gender and age. The distribution of genotypic and allelic frequencies of IL-12A rs568408 demonstrated significant differences between case and control groups. Specifically, the percentages of AA genotype of IL-12A rs568408 was significantly higher among asthmatic patients in Taiwan than healthy controls, compared to GG genotype. No significant difference was observed among the IL-12A rs2243115 and IL-12B rs3212227 genotypes between case and control groups. In addition, the A allele at IL-12A rs568408 was associated with more severe symptoms (P = 0.0085) among asthmatic patients. These results suggest that IL-12A rs568408 may contribute to the etiology and symptoms severity of asthma, indicating its usefulness as a predictive and diagnostic biomarker of asthma.