OBJECTIVES: We examined the impact of functional limitations and functional decline during the first year following breast cancer diagnosis on the risk of mortality from breast cancer and other causes among African-American and white women, respectively.DESIGN: The Health and Functioning in Women (HFW) cohort study.SETTING: Detroit, Michigan, USA.PARTICIPANTS: A total of 162 African-American and 813 white women aged 40-84 years with newly diagnosed breast cancer identified through the Metropolitan Detroit Cancer Surveillance System over a 7-month period between 1984 and 1985 and followed for up to 28 years (median 11 years).OUTCOME MEASURES: Risk of mortality from breast cancer and other causes.RESULTS: Statistically significant increases in the risk of other-cause mortality were found for each unit increase in the number of self-reported functional limitations (HR=1.08, 95% CI 1.03 to 1.14), 0 vs ≥1 functional limitations (HR=1.47, 95% CI 1.13 to 1.91), difficulty in pushing or pulling large objects (HR=1.34, 95% CI 1.04 to 1.73), writing or handling small objects (HR=1.56, 95% CI 1.00 to 2.44), and walking half a mile (HR=1.60, 95% CI 1.19 to 2.14). Functional limitations and functional decline did not explain racial disparities in the survival of this cohort. Functional decline was associated with increased risk of other-cause mortality in women with regional and remote disease but not in women with localised disease. Whereas measures of functional limitation were not associated with breast cancer-specific mortality, each unit of functional decline (HR=1.17, 95% CI 1.05 to 1.31) and decline in the ability to sit ≥1 h (HR=2.06, 95% CI 1.13 to 3.76) were associated with increased risk of breast cancer-specific mortality. Measures of functional decline were associated with increased risk of breast cancer mortality in overweight and obese women, but not in women of normal weight.CONCLUSIONS: Whereas functional limitations were associated with increased risk of other-cause mortality, functional decline was associated with increased risk of breast cancer mortality.