In order to study whether the methacholine inhalation challenge could predict which patients with allergic rhinitis were at risk to develop asthma, we prospectively studied a group of ragweed-sensitive patients over a four to five year period. On the initial study, 16 of 40 patients (40 percent) were found to be hyperresponsive to methacholine. On the follow-up study, three of these 16 patients (19 percent) were found to have developed asthma from one and one-half to five years after the initial testing. Each had greater methacholine responsiveness on repeat study. The degree of methacholine hyperresponsiveness, judged by the PD20, could not predict which of the initial responders would develop asthma. Twenty-four (60 percent) of our patients showed normal responses to methacholine on initial study; none developed asthma and 88 percent remained nonresponders on repeat study. Our study shows that allergic rhinitis patients hyperresponsive to methacholine are at greater risk to develop asthma than those with normal bronchial challenges (p less than 0.05).