In asthma, airflow obstruction is thought to result primarily from inflammation-triggered airway smooth muscle (ASM) contraction. However, anti-inflammatory and smooth muscle-relaxing treatments are often temporary or ineffective. Overproduction of the mucin MUC5AC is an additional disease feature that, while strongly associated pathologically, is poorly understood functionally. Here we show that Muc5ac is a central effector of allergic inflammation that is required for airway hyperreactivity (AHR) to methacholine (MCh). In mice bred on two well-characterized strain backgrounds (C57BL/6 and BALB/c) and exposed to two separate allergic stimuli (ovalbumin and Aspergillus extract), genetic removal of Muc5ac abolishes AHR. Residual MCh responses are identical to unchallenged controls, and although inflammation remains intact, heterogeneous mucous occlusion decreases by 74%. Thus, whereas inflammatory effects on ASM alone are insufficient for AHR, Muc5ac-mediated plugging is an essential mechanism. Inhibiting MUC5AC may be effective for treating asthma and other lung diseases where it is also overproduced.