Background: Accelerometry research suggests physical activity may be associated with treatment toxicity, but generalizability to geriatric oncology is unclear. As many older adults have low levels of physical activity and technology use, further exploration is warranted. This study explores the feasibility of daily physical activity monitoring and the association between step counts and symptoms. Methods: Older adults aged 65+ starting treatment (chemotherapy, enzalutamide/abiraterone, or Radium-223) for metastatic prostate cancer were enrolled in a prospective cohort study. Participants reported step counts (measured via smartphone and a free commercial app) and symptoms (Edmonton Symptom Assessment Scale (ESAS)) daily for their first treatment cycle (~1 month). The feasibility of daily monitoring was assessed with descriptive statistics and thematic analysis. The association between a 15% decline in steps (compared to pre-treatment baseline) and the emergence of moderate-to-severe symptoms and pain (ESAS ≥4) in the next 24 hours was assessed using logistic regression. The utility of a 15% step count decline in predicting the development of moderate-to-severe symptoms and pain was evaluated using sensitivity and positive predictive value (PPV). Results: Of 90 participants, 47 engaged in step count monitoring (mean age 75y; 52.2% participation rate). Daily physical activity monitoring was found to be feasible (90.5% median response rate; 94% retention rate) with numerous patient-reported benefits including increased self-awareness and motivation to engage in physical activity. During the first treatment cycle, instances of a 15% decline in steps were common (n=37, 78.7%), as was the emergence of moderate-to-severe symptoms overall (n=40, 85.1%) and pain (n=26, 55.3%). Overall, daily physical activity did not appear to be significantly predictive of symptoms (OR=1.8, 95% CI=0.42-7.74) overall or pain (OR=0.95, 95% CI=0.22-3.81). Predictive validity estimates revealed reasonable sensitivity for both moderate to severe symptoms overall (sensitivity=88.8%, 95% CI=68.7-95.0; PPV=73.0%, 95% CI=58.7-87.3) but poor PPV for pain (sensitivity=77.8%, 95% CI=58.6-97.0; PPV=37.8%, 95% CI=22.2-53.5). Conclusions: Despite recent studies framing physical activity changes as predictive of toxicity, PPV was only poor to fair. However, daily activity monitoring in older cancer patients appears feasible and may have other uses. Clinical trial information: NCT04193657.