Initially postulated by epidemiological studies, the existence of a relationship between vitamin D and rheumatic diseases has been broadly confirmed by many experimental and clinical studies. In general, the literature points to both a high incidence of hypovitaminosis D in patients with inflammatory/autoimmune rheumatic diseases and a correlation between disease activity/severity and vitamin D levels. Randomized controlled studies have tested the effect of cholecalciferol supplementation (versus placebo) in patients with rheumatic diseases: these have shown significant beneficial effects on both disease activity indices and some clinical outcomes. The complexity of the inflammatory/autoimmune rheumatic diseases and some methodological limitations of published studies to a considerable extent prevent us from making generalizations about cholecalciferol’s therapeutic potential in these conditions. Nevertheless, data from preliminary studies, together with the safety and the low cost of cholecalciferol, strongly support the use of cholecalciferol in patients with these diseases, given also the potential beneficial effects on the bone metabolism.