Purpose: In brain tumours, brain metastases or advanced cancer; treatment with corticosteroids, side effects can add to symptoms. These are best assessed by patients, complementing clinical assessment. We assessed the feasibility and validity of the Dexamethasone Symptom Questionnaire–Chronic (DSQ-Chronic), patient and caregiver versions. Methods: A longitudinal cohort study was conducted, collecting clinician-rated toxicity, performance status, dexamethasone dose and DSQ-Chronic (patient and caregiver versions) at baseline, then 2, 4 and 8 weeks later. Patients had a primary malignant brain tumour, brain metastases, or advanced cancer; Karnofsky Performance Status ≥40 and predicted survival ≥8 weeks. Analysis included questionnaire completion rates, frequency and severity of dexamethasone-attributable side effects, agreement between patient and caregiver ratings, comparison with clinician-rated toxicity and correlation with performance status. Results: Sixty-six patients were recruited (mean age 60 years), with their caregivers. Completion of questionnaires was over 90 % for the dyad at baseline but dropped over time, with caregiver completion rates higher at all timepoints. Agreement between patients and proxies was fair to moderate, and while proxies systematically overestimated symptom severity on DSQ-chronic total scores, the bias was less than 10 points. Patient and clinician agreement was higher for more objective symptoms. Conclusion: The DSQ-Chronic is feasible when the patient is relatively well. As capacity to complete the DSQ-Chronic diminishes, caregivers can be proxy-raters. Clinicians capture corticosteroid toxicities, which may not be obvious to the patient. The DSQ-Chronic, patient and caregiver versions, are useful tools to be used with clinician assessment.