19 Background: The Eastern Cooperative Oncology Group (ECOG) performance status (PS) is one of the most commonly used assessment tools among oncologists and palliative care specialists caring advanced cancer patients. However, the inter-observer difference between the oncologist and palliative care specialist has never been reported. Methods: We retrospectively reviewed the medical records of all patients who were first referred to an outpatient palliative care clinic in 2013 and identified 278 eligible patients. The ECOG PS assessments by palliative care specialists, nurses, and oncologists, and the symptom burden measured by Edmonton Symptom Assessment Scale (ESAS) were analyzed. Results: According to the pairwise comparisons using Sign tests, palliative care specialists rated the ECOG PS grade significantly higher than oncologists (median 0.5 grade, P<0.0001) and nurses also rated significantly higher (median 1.0 grade, P<0.0001). The assessments of palliative care specialists and nurses were not significantly different (P=0.10). Weighted kappa values for inter-observer agreement were 0.26 between palliative care specialists and oncologists, and 0.61 between palliative care specialists and nurses. Palliative care specialists’ assessments showed a moderate correlation with fatigue, dyspnea, anorexia, feeling of well-being, and symptom distress score measured by ESAS. The ECOG PS assessments by all three groups were significantly associated with survival (P<0.001), but the assessments by oncologists could not distinguish survival of patients with PS 2 from 3. Independent predictors of discordance in PS assessments between palliative care specialists and oncologists were the presence of an effective treatment option (odds ratio [OR] 2.39, 95% confidence interval [CI] 1.09-5.23) and poor feeling of well-being (≥4) by ESAS (OR 2.38, 95% CI 1.34-4.21). Conclusions: ECOG PS assessments by the palliative care specialists and nurses were significantly different from the oncologists. Systematic efforts to increase regular interdisciplinary meetings and communications might be crucial to bridge the gap and establish a best care plan for each advanced cancer patients.