The purpose of this study was to evaluate adherence to prescribed opioids in Taiwanese oncology outpatients and to examine the associations between various demographic and medical characteristics and prescribed opioids adherence. Ninety-two outpatients who had taken prescribed opioid analgesics for cancer-related pain at least once in the past week participated in this study. Patients were asked to recall the dose of each opioid analgesic that they had taken in the past 24 hours. Mean adherence rates were calculated for analgesic adherence. For mean adherence rates, all opioid analgesics were converted to morphine equivalents. The results of this study reveal a priority issue of poor opioid analgesic adherence. The adherence rate of 63.6% for the around-the-clock opioid analgesics in this study is well below acceptable levels. Also, an adherence rate of 30.9% for the as-needed opioid analgesics is very low. This study identified that women tend to be less adherent to their prescribed opioid analgesic regimen than men. Findings of this study suggest that to improve pain control, efforts to promote patients' opioid regimen adherence should be given high priority. Clinicians should be particularly aware that there may be some gender difference in adherence to prescribed opioid analgesics. There is a need for better programmatic efforts to improve analgesic adherence.