ABSTRACT: BACKGROUND:Little is known about longitudinal symptom burden, its consequences for well-being, and whether lifestyle moderates the burden in older survivors. METHODS:The authors report on 36-month data from survivors aged ?60 years with newly diagnosed, nonmetastatic breast cancer and noncancer controls recruited from August 2010 through June 2016. Symptom burden was measured as the sum of self-reported symptoms/diseases as follows: pain (yes or no), fatigue (on the Functional Assessment of Cancer Therapy [FACT]-Fatigue scale), cognitive (on the FACT-Cognitive scale), sleep problems (yes or no), depression (on the Center for Epidemiologic Studies Depression scale), anxiety (on the State-Trait Anxiety Inventory), and cardiac problems and neuropathy (yes or no). Well-being was measured using the FACT-General scale, with scores from 0 to 100. Lifestyle included smoking, alcohol use, body mass index, physical activity, and leisure activities. Mixed models assessed relations between treatment group (chemotherapy with or without hormone therapy, hormone therapy only, and controls) and symptom burden, lifestyle, and covariates. Separate models tested the effects of fluctuations in symptom burden and lifestyle on function. RESULTS:All groups reported high baseline symptoms, and levels remained high over time; differences between survivors and controls were most notable for cognitive and sleep problems, anxiety, and neuropathy. The adjusted burden score was highest among chemotherapy-exposed survivors, followed by hormone therapy-exposed survivors versus controls (P < .001). The burden score was related to physical, emotional, and functional well-being (eg, survivors with lower vs higher burden scores had 12.4-point higher physical well-being scores). The composite lifestyle score was not related to symptom burden or well-being, but physical activity was significantly associated with each outcome (P < .005). CONCLUSIONS:Cancer and its treatments are associated with a higher level of actionable symptoms and greater loss of well-being over time in older breast cancer survivors than in comparable noncancer populations, suggesting the need for surveillance and opportunities for intervention.