Tolerability and safety of treatments are important in oncology trials and should be informed by patient assessments. We identified the most relevant patient-reported symptomatic adverse events (AEs) to measure in patients with non-small cell lung cancer (NSCLC) with epidermal growth factor receptor (EGFR) exon 20 insertion mutations. This study selected relevant symptomatic AEs from 78 AEs available in the Patient-Reported Outcomes version of the Common Terminology Criteria for Adverse Events (PRO-CTCAE) measurement system. Initially, symptomatic AEs were selected based on literature and product labeling reviews, and then core sets of symptomatic AEs were identified by patient and clinician interviews. Qualitative and descriptive analyses were performed using the data collected from three iterative rounds of patient interviews. During concept elicitation interviews involving 29 patients, 12 symptomatic AEs were identified and were then adapted into a 25-item PRO-CTCAE form for use in future clinical trials along with commonly used PRO measures. Cognitive interviews showed that the PRO-CTCAE items were easy to answer and appropriate for assessing the patients' experience with symptomatic AEs. This study also assessed disease symptoms, impacts, and overall patient experience. The 25-item PRO-CTCAE form captures the most relevant symptomatic AEs in this patient population, and it is available for future studies. Baseline characterization of AEs associated with this distinct patient group contributes to our broader knowledge about NSCLC and through platforms like Project Patient Voice will expand our understanding of treatment tolerability and safety for NSCLC. Ultimately, this data collection will help inform decision-making for patients, caregivers, healthcare providers, and regulators.