Vitamin D cannot be considered any more as only necessary to prevent rickets or osteomalacia. Calcitriol produced in the kidney is known to have classical endocrine phosphocalcic properties. More recently, vitamin D has been shown to play an important role in reducing the risk of many chronic diseases including type 2 diabetes mellitus, cardiovascular diseases, cancers, and autoimmune and infectious diseases. These effects may be secondary to local production of calcitriol and to its autocrine and paracrine actions on cellular proliferation and differentiation, apoptosis, insulin and renin secretion, interleukin and bactericidal proteins production. These pleiotropic effects are mostly documented by observational and experimental studies or small intervention trials that most often evaluated intermediate parameters. In renal transplant recipients, vitamin D insufficiency, defined as less than 30 ng/ml, is a frequent finding with more than 80% of patients displaying this profile. One may speculate that it could be a part of the explanation for the increased incidence of some complications observed after transplantation. Large intervention trials may therefore be of interest in this specific population.