Background:Vitamin C may influence cancer progression through its antioxidant properties. However, the evidence from observational epidemiologic studies on vitamin C intake and survival following breast cancer diagnosis is not consistent, and the safety of vitamin C supplements following breast cancer diagnosis has not been extensively studied.Methods:Using a food-frequency questionnaire we investigated whether vitamin C intake was associated with survival among 3405 women diagnosed with invasive breast cancer in the Swedish Mammography Cohort.Results:From 1987-2010, there were 1055 total deaths with 416 deaths from breast cancer. Women in the highest quartile of pre-diagnosis vitamin C intake had an adjusted HR (95% CI) of breast cancer death of 0.75 (0.57-0.99) compared with those in the lowest quartile (Ptrend=0.03). There was a borderline significant association between vitamin C intake and total mortality (HR=0.84; 95% CI=0.71-1.00; Ptrend=0.08). Among 717 breast cancer cases for whom post-diagnosis supplement use was available, there was no association between vitamin C supplement use (≈1000 mg) and breast cancer-specific mortality (HR=1.06; 95% CI=0.52-2.17).Conclusion:Our findings suggest that dietary vitamin C intake before breast cancer diagnosis may be associated with breast cancer survival. In addition, post-diagnosis vitamin C supplementation at the level observed in our population was not associated with survival.British Journal of Cancer advance online publication, 4 June 2013; doi:10.1038/bjc.2013.269 www.bjcancer.com.