The importance of vitamin D for calcium absorption and bone health is undisputed. In addition, vitamin D may also be important for more than the skeleton as low serum levels of 25-hydroxyvitamin D (25(OH)D) have been associated with a number of diseases like cardiovascular disease, diabetes, cancer and infections. This is mainly based on observational studies and proof of causal relations from randomized controlled trials (RCTs) are lacking. At present several large RCTs including from 2152 to 25,000 subjects and with cardiovascular disease and cancer as endpoints are ongoing. Results are expected within 3-5 years, and hopefully these studies will give us a definite answer on need for vitamin D supplementation. However, since vitamin D deficiency (serum 25(OH)D <50nmol/L) has not been an inclusion criterion in these studies, there is a risk of a null effect. If so, one has to establish the effects of vitamin D in truly vitamin D deficient subjects, studies that in retrospect obviously should have been the starting point for RCTs on vitamin D and health. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.