Fat-soluble vitamin deficiency and marginal deficiency of this type of vitamin will be discussed. The trias of vitamin A deficiency is composed of nyctalopia, xerophthalmia and hyperkeratosis follicularis. Vitamin D deficiency causes rachitis and osteomalacia. In Japan, vitamin D deficiency is more often caused by impaired vitamin D activation due to renal failure, than by a lack of sunbathing or insufficient ingestion of vitamin D from diet. At present, it is quite rare that the physician encounters patients with deficiency of vitamin E or other vitamins. According to surveys of occult vitamin deficiency, the prevalence of marginal fat-soluble vitamin deficiency was lower than that of marginal water-soluble vitamin deficiency. Marginal vitamin deficiency seems to be absent or very rare among healthy individuals. In patients, however, marginal vitamin deficiency is sometimes observed. Since marginal vitamin deficiency can modify the underlying disease or trigger complications, the physician should take adequate measures to prevent the onset of marginal vitamin deficiency in managing patients with various diseases.