Extensive research on the CYP27B1-encoded 25-(OH)D-1alpha-hydroxylase has contributed much to our understanding about how locally produced 1,25-(OH)2D3 exerts tissue-specific control of cellular growth, differentiation and function. Because many types of epithelial, mesenchymal and immune cells express the 25-(OH)D-1alpha-hydroxylase, many organ functions are necessarily affected by changes in the activity of the enzyme. It is hypothesized that this is likely to occur under conditions of hypovitaminosis D, i.e., at serum 25-(OH)D levels below 30 nM, because extra-renal 25-(OH) D-1alpha-hydroxylase activity is critically limited by the availability of its substrate. This can provide an explanation, on a molecular and cellular basis, for the many observations that significant associations exist between a compromised vitamin D status and the pathogenesis of frequent chronic diseases. In addition to skeletal disorders, vitamin D insufficiency is a risk factor for malignancies, particularly of the colon, breast and prostate gland, as well as for chronic inflammatory and autoimmune diseases (insulin-dependent diabetes mellitus, inflammatory bowel disease, multiple sclerosis, etc.).