Chronic inflammation may be a risk factor for the development and progression of breast cancer, yet it is unknown which inflammatory biomarkers and pathways are especially relevant. The present study included 27,071 participants (mean age = 54.5 years) in the Women's Health Study who were free of cancer and cardiovascular disease at enrollment (1992-1995), with baseline measures of 4 inflammatory biomarkers: high-sensitivity C-reactive protein, fibrinogen, N-acetyl side-chains of acute phase proteins, and soluble intercellular adhesion molecule-1. We used Cox proportional hazards regression models to evaluate associations between baseline concentrations of biomarkers and incident breast cancer, and adjusted for baseline and time-varying factors such as age and body mass index. Self-reported invasive breast cancer was confirmed against medical records for 1,497 incident cases (90% postmenopausal). We observed different patterns of risk depending on the inflammatory biomarker. There was a significant direct association between fibrinogen and breast cancer risk (for quintile 5 vs. quintile 1, adjusted hazard ratio = 1.25, 95% confidence interval: 1.03, 1.51; P for trend = 0.01). In contrast, soluble intercellular adhesion molecule-1 was inversely associated with breast cancer (for quintile 5 vs. quintile 1, adjusted hazard ratio = 0.79, 95% confidence interval: 0.66, 0.94; P for trend = 0.02). N-acetyl side-chains of acute phase proteins and high-sensitivity C-reactive protein were not associated with breast cancer. The complex association of chronic inflammation and breast cancer may be considered when formulating anti-inflammatory cancer prevention or intervention strategies.