It is well known that vitamin D plays a key role in calcium homeostasis and is important for optimal skeletal growth. The major function of vitamin D is to enhance the efficiency of calcium absorption from the small intestine. Most physicians relate vitamin D deficiency to disorders of skeletal muscle. Vitamin D deficiency in children can manifest itself as rickets. In adults, vitamin D deficiency results in osteomalacia. Because most physicians do not appreciate the role of vitamin D deficiency in predisposing the development of cancer, we have written this important report as a wake-up call to physicians and other healthcare workers in documenting the relationship of vitamin D deficiency and cancer. Epidemiological data show an inverse relationship between vitamin D levels and breast cancer incidence. In addition, there is a well-documented association between vitamin D intake and the risk of breast cancer. Low vitamin D intake has also been indicated in colorectal carcinogenesis. A vitamin D deficiency has also been documented in patients with prostate cancer, ovarian cancer, as well as multiple myeloma. Larger randomized clinical trials should be undertaken in humans to establish the role of vitamin D supplementation in the prevention of these cancers.