Given the high rates of relapse among patients with opioid use disorder (OUD), it is crucial to identify modifiable risk factors for negative treatment outcomes. Anxiety sensitivity (AS) is 1 such risk factor that may be associated with negative OUD treatment outcomes. The present study examined the potential impact of AS on the withdrawal process, subsequent treatment engagement, and relapse among individuals with OUD. Adults undergoing inpatient detoxification (N = 90) completed self-report and researcher-administered questionnaires on Day 4 of a 5-day buprenorphine-assisted detoxification protocol, and 1 month later a follow-up evaluation assessed treatment engagement and relapse. Although 68% of the sample engaged in subsequent treatment, 76% demonstrated poor adherence. Over half the sample (57%) reported opioid relapse 1 month later. Results revealed that greater AS and younger age predicted greater fear of withdrawal during detoxification. Contrary to the research hypotheses, AS was not a significant predictor of other treatment outcomes; rather, fear of withdrawal and prior number of opioid detoxifications predicted greater subjective withdrawal severity. During detoxification, younger age was related to greater cravings, and being a male was associated with a higher likelihood of receiving prescription anxiolytics. Following detoxification treatment, referral to residential treatment predicted greater treatment engagement, whereas greater opioid craving, number of days in an uncontrolled environment, and any nonopioid substance use postdischarge predicted greater opioid relapse. Failure to find a relationship between AS and the withdrawal process is potentially a function of the buprenorphine protocol. Overall, findings may have important implications for the treatment of OUD.