We retrospectively investigated the use of oral hydromorphone for cancer pain. Nineteen patients treated for cancer pain with oral hydromorphone were reviewed in this study. Cancers had occurred in the gastrointestinal (n=4), lung(n=3), breast(n=2), bone and soft tissue(n=2), hematological(n=2), and others(n=6). The administered opioids before switching to hydromorphone were morphine, oxycodone, and tapentadol. The mean oral morphine equivalent daily dose (OMEDD)was 89.3 mg. The average dose of hydromorphone administered was 16.4 mg/day, and average NRS 10(numerical rating scale: 0-10)scores of cancer pain before and after switching were 4.1 and 3.8, respectively, showing no significant differences. In this study, switching from other opioids to oral hydromorphone was feasible with an approved conversion ratio, ie, an oral hydromorphone-to-oral morphine ratio of 1:5. No severe adverse effects were observed. The oral hydromorphone extended-release formulation was administered every 24 h, as a tiny tablet formulation that is preferable owing to easy administration and adherence.