This study examines the impact of smoking, body mass index, alcohol consumption, hormone replacement therapy, and physical activity on all-cause mortality among 528 Danish women diagnosed with primary breast cancer. Participants were women enrolled in the Copenhagen City Heart Study. Prospective self-reported exposure information was collected from four points of follow-up in 1976–1978, 1981–1983, 1991–1994, and 2001–2003. Kaplan–Meier survival curves and multivariate Cox regression analyses were performed adjusting for age, disease stage, adjuvant treatment, menopausal status, parity, alcohol intake, smoking, physical activity, body mass index, and hormone replacement therapy. The study shows that smoking for total mortality [hazard ratio, 1.16; 95% confidence interval, 1.05–1.29] and obesity for both total mortality (1.61; 1.12–2.33) and breast cancer-specific mortality (1.82; 1.11–2.99) were significantly associated with decreased survival after breast cancer diagnosis. A moderate alcohol intake of 1-6 units/week (0.85; 0.64–1.12), 7–14 units/week (0.77; 0.56–1.08), and treatment with hormone replacement therapy (0.79; 0.59–1.05) were less than 1, but not statistically significantly associated with prolonged survival. A moderate physical activity of 2–4 h/week (1.07; 0.77–1.49) and a high physical activity of more than 4 h/week (1.00; 0.69–1.45) showed no association with survival after breast cancer diagnosis.