OBJECTIVE: This study aimed to clarify the physical activity level of patients with upper gastrointestinal cancer during outpatient chemotherapy and the factors associated with decreased physical activity levels after drug administration. METHODS: In this cross-sectional study, activity intensity and steps were measured using an accelerometer in 39 patients with upper gastrointestinal cancer for 1 week before and after drug administration. Furthermore, the participants responded to a questionnaire on their lifestyles. RESULTS: No significant differences in steps and activity intensity were found before and after drug administration, and many participants had low activity levels. Logistic regression analysis showed that the Geriatric 8 scores and domestic roles were positively associated with higher activity levels after drug administration, whereas total bedtime showed a negative association. A predictive score for low activity was calculated from the three associated factors, and receiver operating characteristic analysis was conducted, resulting in an area under the curve of 0.90. CONCLUSIONS: Physical activity may be low in patients with upper gastrointestinal cancer during outpatient chemotherapy. To maintain and promote physical activity, the results suggest the need to predict those who become less active after treatment and to support them by focusing on their domestic roles and total bedtime while considering their general condition. Our newly proposed predictive score can objectively identify patients with lower physical activity, regardless of the nurse's experience and ability, and improve patients' support during chemotherapy, even in the limited time available as outpatients.