Background Signing advance directives (ADs) ensures that terminally ill patients receive end-of-life care, according to their wishes, thereby promoting human dignity and sparing them from unnecessary suffering. Despite the enactment of the Hospice Palliative Care Act in Taiwan in 2000, the completion rates of ADs have been found to be low among patients with chronic illness conditions. To date, limited existing research is available regarding the factors associated with AD completion in terminally ill patients in Taiwan. To explore signed AD characteristics, compare differences in signing ADs between patients with and without cancer, and examine the factors associated with signing ADs in terminally ill patients. Methods A nationwide study was conducted using data collected via a retrospective review of medical death records from 18 randomly selected hospitals in the northern, central, and southern parts of Taiwan. We collected 200 records, including both cancer and non-cancer-related deaths, from each hospital. Univariate and multivariate logistics regressions were conducted to examine factors associated with signing advance directives among all patients- with and without cancer. Results Among the 3004 reviewed medical records, 79% had signed ADs, with most (95%) being signed by patients’ caregivers. A higher education level (OR = 1.52, 95% CI = 1.10, 2.08, p = 0.010); cancer diagnosis (OR = 2.37, 95% CI = 1.79, 3.16, p < 0.001); having family members (OR = 5.62, 95% CI = 2.95, 10.69, p < 0.001), care homes (OR = 4.52, 95% CI = 1.97, 10.38, p < 0.001), friends, or maids (OR = 3.82, 95% CI = 1.76, 8.29, p = 0.001) as primary caregivers; and patients knowing about their poor prognosis (OR = 15.39, 95% CI = 5.66, 41.83, p < 0.001) were associated with a higher likelihood of signing ADs. Conclusions Patients with non-malignant chronic illnesses were less likely to have ADs signed by either patients or family caregivers than those with cancer, with the lowest likelihood observed in patients with cardiovascular diseases. Whenever possible, primary caregivers should be involved in discussing ADs with patients, and the importance of truth telling should be reinforced. Following these principles, each patient’s end-of-life care preferences can be respected, thereby promoting quality of care before the patient’s death.