CONTEXT: In surgical and clinical oncology, there is a growing need for patient-centered outcomes that are responsive, meaningful, and fit for purpose. OBJECTIVES: The aim of this study was to validate physical activity (PA) monitoring as a responsive outcome measure at different stages of disease and treatment, by verifying correlations between PA, performance score, and quality of life (QoL). METHODS: Daily life PA of 162 cancer patients, monitored by a device that records time sitting/lying, time standing, time walking, number of steps taken, and walking cadence, was compared with 20 healthy volunteers. In a subgroup of patients, functional status and QoL were assessed using the World Health Organization/Eastern Cooperative Oncology Group and the Karnofsky Performance Status scores and the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire-C30 (EORTC QLQ-C30) questionnaire. RESULTS: The PA of patients with resectable gastrointestinal cancer did not differ significantly from controls. In contrast, patients with advanced cancer took 45% fewer steps and spent an extra 2.8 hours/day lying/sitting (P=0.001). Patients undergoing neoadjuvant chemotherapy and surgery (5-6 weeks after operation) experienced a similar reduction in PA. There were significant correlations between PA and the physical and role domains as well as fatigue subscale of the EORTC QLQ-C30 scale. CONCLUSION: Objective PA scores correlate significantly with disease stage, functional status, and QoL of patients with cancer. Therefore, activity monitoring can make meaningful objective estimates of patient function in response to cancer and its treatment and may provide surrogate outcomes of QoL.