Background: Systematic review of electronic patient reported outcomes (ePRO) has been shown to improve quality of life and overall survival in clinical trial. We previously demonstrated feasibility of ePRO across Dana-Farber Cancer Institute (DFCI). We sought to examine the distribution and frequency of first symptomatic adverse events (SAEs) among ePRO responders in ambulatory oncology practice. Methods: The ePRO tool uses the validated NCI developed Patient Reported Outcomes – Common Terminology Criteria for Adverse Events (PRO-CTCAE) instrument to assess attributes of 15 core SAEs (fatigue, insomnia, general pain, decreased appetite, nausea, vomiting, constipation, diarrhea, shortness of breath, numbness and tingling, rash, concentration, fever, anxiety, sadness) selected by clinician stakeholders and deployed via any internet-enabled device once every 7 days. Responses are viewable in the EHR, scored 0 to 3 using an algorithm, with scores of 3 highlighted to indicate severe grade SAEs. Results: We examined the distribution and frequency of the first 5183 unique ePRO reports for unselected patients seen in the medical, radiation and surgical oncology outpatient clinics of four pilot multidisciplinary clinics (Breast, Genitourinary, Gastrointestinal and Head and Neck) between September 2018-December 2019. Twenty one percent of eligible patients responded to ePRO (5183 of 26,084). Most respondents were female (59%), Caucasian (89%), and age 50-69 years (56% compared to 16% age <50 years, 28% age ≥70; range 19-98 years). The frequency of grade 3 SAEs was pain (10%), fatigue (6%), insomnia (4%), constipation (3%), numbness and tingling/concentration/anxiety/decreased appetite (2%), diarrhea/shortness of breath/sadness (1%), and rash/fever/nausea/vomiting (none) (Table). Conclusions: We observed a consistent distribution of SAEs across cancer types, age and sex. The most frequently reported SAEs are those clinicians struggle to treat with medications - pain, fatigue, insomnia and anxiety. Research to develop effective strategies to address this constellation of SAEs should be prioritized.