BACKGROUND: Ambient ultraviolet (UV) radiation has been increasing due to climate change. While this may result in adverse health consequences such as an increased incidence of skin cancer, UV radiation is also a source of vitamin D, which has been hypothesized to be protective for breast cancer risk. METHODS: Using a spatiotemporal kriging model, we estimated residential UV exposure levels for the enrollment addresses (2003-2009) of breast cancer-free women aged 35-74 years participating in the Sister Study and living in the contiguous United States (N = 48,450). Cox proportional hazards models were used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (95% CIs) for the risk associated with UV exposure levels (mW/m^(2)) categorized in quintiles. We examined the association for breast cancer overall (invasive and ductal carcinoma in situ) and by estrogen receptor (ER) status of the tumor. We considered effect modification by regular (≥4 times/week) vitamin D supplement use. RESULTS: Over a median of 10.5 years of follow up, 3,510 incident breast cancer diagnoses were reported. We found no evidence of an association between living in areas with higher levels of UV radiation and overall breast cancer risk (HR(Q5 vs. Q1) = 1.00, 95% CI: 0.90, 1.11). Higher UV levels were inversely associated with the risk of ER- breast cancer (HR(Q5 vs. Q1) = 0.73, 95% CI: 0.55-0.99), but not ER+ (HR (Q5 vs. Q1) = 1.04, 95% CI: 0.92-1.18). For ER- breast cancer, the inverse association was only evident in women who did not regularly take vitamin D supplements (HR(Q5 vs. Q1) = 0.52, 95% CI: 0.33-0.81) compared with those who did regularly take vitamin D supplements (HR(Q5 vs. Q1) = 1.02, 95% CI: 0.68-1.54; p-for-heterogeneity = 0.12). CONCLUSIONS: The findings from this study support a role for UV exposure and vitamin D in the etiology of ER- breast cancer.