This study examined whether, after a breast cancer diagnosis, high intake of animal fat was associated with increased breast cancer mortality and high intake of fiber was associated with decreased breast cancer mortality. Participants were 3,846 US female nurses diagnosed with stages I-III breast cancer between 1976 and 2001 and followed until death or May 2006. Breast cancer mortality was calculated according to dietary intake quintiles first assessed at least 12 months after diagnosis and was cumulatively averaged and updated. There were 446 breast cancer deaths. In simple models adjusted for time since diagnosis, age, and energy intake, animal fat intake was associated with increased breast cancer death, and cereal fiber intake was associated with reduced breast cancer death. However, no associations were found in fully adjusted models: for animal fat, the relative risks for increasing quintiles were 1.00, 0.89, 0.86, 0.85, and 0.89 (95% confidence interval: 0.61, 1.28), P = 0.68; for cereal fiber, they were 1.00, 0.95, 0.76, 0.81, and 1.00 (95% confidence interval: 0.71, 1.40), P = 0.59. Results of simple models adjusted additionally for physical activity were similar to those for full multivariate models. Results show that physical activity strongly confounds the association between diet and survival.