Recent studies have shed new light on the functions of vitamins K and D. It has also been suggested that these vitamins have synergistic effects on certain metabolic processes (e.g. the formation of bone tissue). The serum levels of vitamin D are positively correlated with the degree of bone mineralisation and negatively correlated with serum parathyroid hormone levels. The mechanism underlying the effects of vitamin K on the skeletal system is associated with increased bone mineralization (carboxylation of Gla-protein osteocalcin) and decreased bone resorption due to the inhibition of osteoclast activity. Recently published studies have shown a relationship between the development of central nervous system diseases and low levels of vitamins K and D. It has been shown that deficiency of these vitamins may influence the development of dementia. Vitamin K has been shown to be involved in regulating the activity of sulfotransferases and Gas6 protein responsible for the accelerated growth of Schwann cells. There is a negative correlation between vitamin K intake and the risk of vascular calcification and mortality due to cardiovascular diseases. A close relationship was demonstrated between the activity of vitamin D and the presence of calcifications in the blood vessels in the course of a number of diseases such as atherosclerosis, osteoporosis and chronic kidney disease. It was also shown that a concomitant administration of vitamins K and C results in the induction of oxidative stress, leading to an anticancer effect without the use of chemotherapeutics. Squamous tumour cell line was most susceptible compared to endometrial and breast cancer cells. Antitumor effects of vitamin D are related to the mechanisms underlying the inhibition of proliferation, the activation of apoptosis and the inhibition of angiogenesis. The presence of an active, non-mutated vitamin D receptor is essential for an effective action of vitamin D. Increased vitamin D receptor expression is observed in the basal cell carcinoma of the skin and cervical cancer compared with healthy tissue.