BACKGROUND: Decline in upper-body function and development of upper-body symptoms are adverse effects of breast cancer therapy and may affect functional independence, particularly among older survivors. The long-term risks and predictors are poorly understood. OBJECTIVE: To characterize the risk of decline in upper-body function and development of symptoms over 4 years of follow-up. DESIGN: We used a prospective cohort design. PARTICIPANTS: Six hundred and forty-four early stage breast cancer patients 65 years old or older at surgery enrolled in Rhode Island, North Carolina, Minnesota, and Los Angeles between 1996 and 1999. MEASUREMENTS: Upper-body function and symptoms were self-reported at baseline, 6, 15 months, and annually thereafter to 51 months after surgery. RESULTS: One half of the participants had a decline in upper-body function and one-quarter developed upper-body symptoms. Breast cancer patients were 5-fold more likely to have a decline in upper-body function over 4 years of follow-up than a similar cohort without breast cancer. Better baseline mental health protected against a decline in upper-body function (odds ratio [OR]=0.93, 95% confidence interval [CI] 0.88 to 0.97 for 8-point higher mental health index). Baseline obesity (OR for body mass index [BMI] > or =30 kg/m2 vs <30 kg/m2=2.5, CI=1.6 to 4.0) and axillary node dissection (OR for axillary dissection vs not=3.9, CI=1.1 to 14) predicted the development of upper-body symptoms. CONCLUSIONS: Primary care physicians should address upper-body function and symptoms with older breast cancer patients, and inform them that these complications of breast cancer treatment are common.