Background: Patients with advanced cancer undergoing chemotherapy experience significant symptoms and declines in functional status, which are often associated with poor outcomes. Remote monitoring of PROs (symptoms) and step counts (functional status), may proactively identify patients at risk of hospitalization or death. Our objectives were to evaluate the association of 1) longitudinal PROs with step counts, and 2) PROs and step counts with hospitalization or death. Methods: The PROStep randomized trial (NCT04616768) enrolled 108 patients with advanced gastrointestinal (GI) or lung cancers undergoing cytotoxic chemotherapy at a large academic cancer center. Patients were randomized to weekly text-based monitoring of 8 PROs (each measured on a 0-4 scale) + continuous step count monitoring via Fitbit, vs usual care. This pre-planned secondary analysis included 57 of 75 patients randomized to the intervention who had PRO and step data. We analyzed associations between PROs and mean daily step count, and associations of PROs and step counts with the composite outcome of hospitalization or death, using bootstrapped generalized linear models to account for longitudinal data. Results: Among 57 evaluable patients, mean age was 57 years, 24 (42%) were female, 49 (86%) were White, 43 (75%) had advanced GI cancer, and 14 (25%) had advanced lung cancer. A 1-point weekly increase (on a 32-point scale) in aggregate PRO score was associated with 247 fewer mean daily steps (95% CI -277, -213, p < 0.001). PROs most strongly associated with step count decline were patient-reported activity level (daily step change -892), nausea (-677), and constipation (-524). 25 patients (44%) were hospitalized (n = 21) or died (n = 8). A 1-point weekly increase in aggregate PRO score was associated with 20% greater odds of hospitalization or death (adjusted odds ratio [aOR] 1.2, 95% CI 1.1, 1.4, p = 0.008). PROs most strongly associated with hospitalization/death were pain (aOR 3.2), decreased activity (aOR 3.2), dypsnea (aOR 2.6) and sadness (aOR 2.1). A decrease in 1000 steps was associated with 16% greater odds of hospitalization or death (aOR 1.2, 95% CI 1.0, 1.3, p = 0.03). Compared to baseline, mean daily step count decreased 6.7%, 8.5%, 16.2% in the 3, 2, and 1 weeks prior to hospitalization/death. Mean aggregate PRO score increased by 11.0%, 25.3%, 36.4% in the 3, 2, and 1 weeks prior to hospitalization/death. Conclusions: In this secondary analysis of a randomized trial, increased PRO-based symptom burden was associated with lower daily step count. Higher symptom burden and decreased step count were independently associated with and predictably worsened close to hospitalization or death. Future interventions should leverage longitudinal PRO and step data to identify risk for poor outcomes. Clinical trial information: NCT04616768.