Background: Physical function is important for defining treatment strategies in patients with cancer and can be estimated using patient-reported outcomes (pros). Although pros are subjective, physical activity and fitness can be tested objectively with adequate, but more labour-intensive methods that are rarely used in daily clinical practice. To determine whether pros for physical function (pro-pf) accurately predict physical function, we studied their interrelationships with objective measures of physical activity and fitness in patients with cancer who had completed cancer treatment, including adjuvant or neoadjuvant chemotherapy or autologous stem-cell transplantation. Methods: Baseline data from the react (Resistance and Endurance Exercise After Chemotherapy) and exist (Exercise Intervention After Stem-Cell Transplantation) studies were evaluated. In those studies, the effects of an exercise intervention on physical fitness, fatigue, and health-related quality of life in patients with cancer shortly after completion of chemotherapy or stem-cell transplantation were studied. Interrelationships between pro-pf (physical function subscale of the European Organisation for Research and Treatment of Cancer 30-question core Quality of Life Questionnaire), physical activity (accelerometer), and cardiorespiratory fitness (peak oxygen uptake) were assessed using univariable and multivariable multilevel linear mixed-model analyses. Results: After adjustment for age, sex, and body mass index, the pro-pf was significantly associated with physical activity (β = 1.75; 95% confidence interval: 1.08 to 2.42) and cardiorespiratory fitness (β = 0.10; 95% confidence interval: 0.06 to 0.13). Standardized coefficients were 0.28 and 0.26 respectively, indicating a weak association. Conclusions: The pro-pf is only weakly associated with objective physical activity and fitness evaluation in patients after curative treatment for cancer. The pro-pf cannot, therefore, be used in clinical practice as a substitute for objective measures of physical function.