In females, breast cancer is the most common cancer along with being one of the leading causes of cancer deaths worldwide. The known risk factors associated with breast cancer are both non-modifiable and modifiable. Factors like Vitamin D and mammographic density are few of the modifiable factors in breast cancer prevention which have recently been emphasized upon. Vitamin D is the talk of all researchers with more emphasis on its non-bone effects. In the recent past, it has been found to modulate breast epithelial cell proliferation, for both normal and cancerous cells. Many studies have endorsed the fact that vitamin D may be associated with reduced breast cancer risk. The hypothesis was ―vitamin D deficiency is associated with epithelial cell proliferation and mammographic density.‖ The primary objective of this thesis was to evaluate the effect of vitamin D deficiency and its association with epithelial cell proliferation and mammographic density in the reproductive age group. Mammographic density was estimated using the fullyautomated software Volpara. Secondary objectives were to find out the effect of the enzyme 1 alpha Hydroxylase, DNA quantification and Vitamin D receptor gene polymorphism (fok1 and Apa1) by using PCR/RFLP. The epithelial cell proliferation was studied by performing FNAC on palpable lumps and utilizing ―countess‖ to count the viable and non-viable cells present in the tissue. This was a prospective, open label, clinical trial. The total cases were recruited from the surgical OPD of Patel Hospital, Karachi (n=350) and were supplemented with vitamin D. The study was carried out from June 2013-March 2015. The results were favorable in most patients, with general increase of vitamin 25(OH) D levels after supplementation (baseline mean ± SD: 9.88 ± 7.3; median 7.0; range 3– 49.8) to 68.3 + 25.5; 66; 30.1-150, p< 0.001), a mean difference of 58.4 between vitamin D level at baseline and 12 months. There was a weak, positive Spearman''s rankxx order correlation between 1 alpha Hydroxylase at 12 months and vitamin D at baseline; 12 months (rs= 0.169, p= 0.126,rs= 0.079,p=0.480).A moderate negative correlation was found between vitamin D baseline – Volumetric breast density (VBD) baseline and vitamin D 12 months – VBD 12 months (rs= -0.245, p= <0.001,rs= -0.289, p=0.193) respectively. No statistically significant association was found between vitamin D at baseline and BI-RADS at baseline (p= 0.126). A Kruskal-Wallis H test showed that there was statistically significant difference in VBD at baseline between the different BI-RADS groups, χ2(3) = 169.4, p<0.001, with a mean rank of 6 for BI-RADS 1, 36.11 for BIRADS 2,105.67 for BI-RADS 3, and 176.55 for BI-RADS 4. A negligible or no correlation was found between vitamin D baseline with levels of proliferation in breast cells (total cell count rs= -0.020, p=0.906; dead cell count rs= -0.005, p=0.979; viable cell count rs= -0.072, p= 0.672). VBD was moderate and negatively correlated with dead cell count (rs= -0.436, p=0.018) while weak negative correlation was observed between VBD and total cell count (rs=-0.130,p=0.502). The conclusion is that an optimal level of vitamin D has to be maintained ;if not then vitamin D level falls to its original deficient level in two months time. There is a need to establish a universal definition of vitamin D deficiency to allow better comparability of studies at the global level. Only through the identification of modifiable risk factors associated with this disease, can effective cancer prevention be realized.