It has been shown during the last years that transplant recipients may develop vitamin D deficiency.1,2 Although the reasons for relatively low vitamin D levels in transplant recipients are thought to be multifactoral, I believe that a lack of UV exposure may be the major cause. Exposure to UV is the main reason for the development of nonmelanoma skin cancer.3 Because immunosuppressive therapy markedly increases the risk of developing UV-induced skin cancer, it is extremely important for transplant recipients to protect themselves against UV exposure. On the other hand, 90% of all vitamin D is formed within the skin through the action of the sun. This is a serious problem, as a connection between vitamin D deficiency and severe health problems, including various types of cancer (eg, colon, prostate, and breast cancer), has been suggested in a large number of studies.4,5 As a consequence, the association between vitamin D deficiency and various internal malignancies has now opened a debate among clinicians regarding the necessity to detect and treat vitamin D deficiency in transplant recipients more agressively. What is the rationale for an association between vitamin D deficiency and an increased risk of certain types of cancer? A negative association has been reported between increased risk of dying of various internal malignancies (eg, breast, colon, prostate, and ovarian cancer) and living at decreasing degrees of latitude.4,5 Additionally, a correlation of this latitudinal association with decreased vitamin D serum levels has been shown.4 It has now been demonstrated that, in contrast to earlier assumptions, skin, prostate, colon, breast, and many other tissues express an enzyme (25-hydroxyvitamin D [25(OH)D]-1α-hydroxylase) that converts 25(OH)D to its active form, 1,25(OH)2D.6,7 Therefore 1,25(OH)2D is now not exclusively considered a calciotropic hormone but also a locally produced regulator of cell growth.6,7 Consequently, recently published studies point to a protective effect of locally produced vitamin D in the pathogenesis of various malignancies.