The physiologically active metabolite of vitamin D3, 1 alpha,25-dihydroxycolicalciferol [1,25(OH)2D3, calcitriol] has achieved the status of a hormone. It is believed to mediate its effects by binding to a specific receptor which belongs to the family of nuclear receptors for glucocorticoids, estrogens, thyroxine, and retinoid acid. It has been discovered that 1,25(OH)2D3 has the ability to regulate growth and differentiation in many cell types, including cancer cells, epidermal keratinocytes, and activated lymphocytes. This has set the stage for the development of a new class of compounds with potential usefulness in hyperproliferative and immune-mediated diseases. Ideally, such agents should possess potent effects as regulators of cell proliferation and differentiation at concentrations well below those that may induce side effects related to the classical vitamin D activity on calcium absorption and bone mineralization. In addition to 1,25(OH)2D3, the synthetic vitamin D3 analogues 1 alpha-OH-D3, 1,24(OH)2D3, and calcipotriol have undergone clinical evaluation. Calcipotriol has been studied most extensively. Compared with 1,25(OH)2D3, calcipotriol is about 200 times less potent in its effect on calcium metabolism although similar in receptor affinity. In double-blind, placebo-controlled multicenter studies, topical calcipotriol has been shown to be both efficacious and safe for the short- and long-term treatment of plaque-type psoriasis. Because some of the novel vitamin D analogues are potent regulators of cell growth and immune responses, they may be of potential interest in the treatment of ichthyoses, cancer, and autoimmune diseases.