The recent identification of an expanded role for vitamin D action, beyond its traditional actions in mineral metabolism and the novel investigational approaches as well, has opened new therapeutic avenues for both the clinician at bedside and the scientist. This is the rationale of dedicating a special issue on this hot topic.

A new spectrum of vitamin D biological activities including important aspects on cellular proliferation, differentiation, and the immune system has been recently identified. Relevant is also the interaction of vitamin D with other kidney hormones such as renin and erythropoietin; this topic is specifically addressed by this paper. Indeed, the administration of vitamin D analogues has been associated with an improvement in anaemia and reduction in erythropoiesisstimulating agents (ESA) requirements. Furthermore, vitamin D deficiency could contribute to the inappropriately activated or unsuppressed renin-angiotensin-aldosterone system (RAAS) in pathologic conditions. Experimental data show that vitamin D may also interfere with the compensatory increase in renin synthesis occurring during chronic administration of anti-RAAS agents. In this context, use of vitamin D may be therefore suggested in patients treated with anti-RAAS but not reaching the safe threshold level of proteinuria. As further support to the hypothesis on the use of vitamin D analogues as antiproteinuric agents in proteinuric chronic kidney disease (CKD) are the data evidencing its anti-inflammatory effects. In this special issue, J. Egido et al. provided novel insights into the association between vitamin D deficiency and the activation