Interest in vitamin D status remains high amongst the public and medical profession alike stimulating a surge in requests for clinical laboratories to measure patient serum 25-hydroxyvitamin D levels. It is stimulated by reports that a low vitamin D status is common and is associated with an increased incidence of a variety of diseases. Questions facing the clinical laboratory providing this service include: How reliable are the various serum 25-hydroxyvitamin D assays that are available to the clinical laboratory and which patient groups benefit most from this test? In the past confidence in the quality of serum 25-hydroxyvitamin D assays has been questioned. An internationally accepted reference measurement procedure and reference standards for serum 25-hydroxyvitamin D assays are now available. Restandardization of routine automated assays has been implemented for a number of assays demonstrating considerable improvement in the accuracy of these assays for most patient groups. However recent data suggest that for some patient groups inaccuracy is still a significant clinical problem. The highest levels of evidence indicate that the elderly with a low vitamin D status benefit from vitamin D and dietary calcium supplementation to reduce risks of premature mortality, falls and fractures. There is no agreement that screening for vitamin D deficiency in the general population is cost effective. However there is agreement that a variety of patients groups can benefit by correcting their vitamin D deficiency. Interpretation of serum 25-hydroxyvitamin D levels remains highly controversial although considerable advances have been made elucidating the physiology of vitamin D metabolite homeostasis.