Background: Treatment options in 1 st line metastatic breast cancer (MBC) vary and may include chemotherapy, targeted, and hormone therapy. Some patients (pts) do not complete therapy as planned, perhaps due to treatment related toxicities. The current study examined the association of symptom burden with early treatment discontinuation (ETD) of 1 st line therapy of MBC in real-world settings. Methods: Data were abstracted from medical records of pts at 9 community oncology practices. Eligible pts had stage IV breast cancer with start of 1 st line therapy on 1/2004 to 6/2012, were ≥ 18 years, and had ≥ 1 Patient Care Monitor (PCM) survey during 1 st line. The PCM is an 86-item survey of cancer-related symptoms collected as part of routine clinical care. Age, race, HER2 status, hormone status, oral and infused agents, dates of diagnosis, treatment, progression, and death were recorded. ETD was defined by direct indication of early stopping in the medical record, or by treatment duration ≤ 6 weeks without evidence of disease progression. Cox regression of ETD with time varying covariates was used to examine the impact of 23 separate symptoms as well as an overall composite symptom burden score based on pt responses on each symptom. Results: 797 pts were included, with mean age of 58.4 years, 62.1% White; with 340 on Chemotherapy (CT), 349 on CT + Targeted therapy (T) and 108 on Hormone therapy only (H). Overall, ETD occurred in 95 (11.9%) pts, with rates highest among CT (15.3%), followed by T (10.0%) and H (7.4%). Cox regression showed that 21 of 23 symptoms each increased the risk of ETD. In the composite symptom burden score (median 7.1; range 0 - 21) analysis, overall symptom burden was found to be significant (HR = 1.124, p < 0.0001), indicating a 12.4% increased risk of ETD with each additional symptom. Pts with 10+ symptom score had a significantly increased risk of ETD (HR = 3.09, p < 0.0001) compared to pts with < 5 symptom score. Pts with 15+ symptom score had the highest risk of ETD (HR = 5.75, p < 0.0001). Conclusions: Pts treated with CT for 1 st line MBC had the highest rate of ETD. The likelihood of ETD increased as the number of symptoms increased. Future research is needed to evaluate ETD on pt outcomes, including overall survival.