Aim. To investigate whether breast cancer incidence increases with increasing latitude in mainland Australian women. Methods. A cross-sectional study of female breast cancer and cutaneous melanoma incidence 2002–2006 by 5-year age group and local government area. Latitude, Accessibility/Remoteness Index of Australia (ARIA), and Index of Relative Socioeconomic Disadvantage (IRSD) were assigned to local government areas. Latitude was grouped into bands (≤27°S; >27–30°S; >30–33°S; >33–36°S, and >36°S), and IRSD was divided into quintiles and ARIA into four categories. Breast cancer rates were age standardized using the direct method. The joint effects of latitude, age, IRSD, and ARIA on incidence of breast cancer and cutaneous melanoma were assessed using multiple logistic regressions. Results. At latitudes south of 30°S, rates of breast cancer were over double that north of 27°S (76.4 versus 160.2–176.5). Age-adjusted odds ratios of breast cancer were increased in all latitudes south of 30°S compared with north of 27°S within each IRSD and ARIA category (all ). After adjusting for age, IRSD, and ARIA, the odds ratio of breast cancer south of 30°S was 1.92 (95% CI 1.84–2.09; ), whereas cutaneous melanoma was 0.65 (95% CI 0.61–0.68; ) times north of 30°S. Discussion. Increasing latitude is positively associated with breast cancer and negatively associated with cutaneous melanoma incidence. These findings support suggestions that increased risk of breast cancer might be explained by lower ultraviolet radiation-induced vitamin D synthesis. 1. Introduction Ecologic studies show that the incidence and mortality of breast cancer increase from the equator (latitude 0°) as latitude increases towards the earth’s poles (90°) [1–3]. Similarly, the rates of breast cancer have been shown to be inversely associated with the level of ultraviolet (UV) radiation from sunlight [4–9]. For most individuals, solar UV radiation in the B spectrum is the primary source of vitamin D [10]. Exposure of the skin to UV radiation between the wavelengths of 290 and 330？nm causes photolysis of 7-dehydrocholesterol to form previtamin D3. In a heat-induced process, previtamin D3 is then converted to vitamin D3, which is subsequently hydroxylated in the liver and the kidneys or in local tissue such as breast, to form the active form of the vitamin, 25-hydroxyvitamin D3 [10]. Geographic latitude is an important determinant of cutaneous vitamin D synthesis, as it influences the amount of UV radiation reaching the skin [11]. An increase in latitude causes the solar zenith angle, the angle
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