There has been a poor consensus in defining normal levels of 25(OH) D. It has been traditionally recognized that 25(OH)D serum levels below 5-7 ng/ml induce osteomalacia, serum levels below 10-12 ng/ml induce secondary hyperparathyroidism and osteoporosis, and serum levels above 18-20 ng/ml are usually considered normal or adequate. Due to the results obtained in several studies, a more functional classification has recently been proposed defining serum 25(OH)D levels > 40 ng/ml or > 100 nmol/l as "desirable", serum levels between 20 and 40 ng/ml or 50 and 100 nmol/l as hypovitaminosis D, levels between 10 and 20 ng/ml or 25 and 50 mmol/l as vitamin D insufficiency and 25(OH)D levels below 10 ng/ml or 25 nmol/l as deficient. These new cut-off levels, suggest that, in the past, we had been using a wrong statistical approach for defining "normal serum 25(OH)D levels". In agreement with this new classification, in a recent study conducted in a random sample of our population, a high prevalence of low levels of 25(OH)D and secondary hyperparathyroidism was found. In our study, only in those people having "excellent" renal function, representing only 15% of the sample (serum creatinine < 1 mg/dl in men and < 0.8 in women, mean age of 68 years) hyperparathyroidism was not diagnosed despite observing 25(OH)D serum levels around 18-30 ng/ml or 45-75 nmol/l). In the remaining people (85% of the sample), who showed the expected serum creatinine increments according to their age, secondary hyperparathyroidism was avoided only if the serum 25(OH)D levels were higher than 30 ng/ml or 75 nmol/l. These remarkable findings demonstrate the importance of maintaining higher 25(OH)D levels--in addition to normal calcitriol levels--in order to avoid stimulation of the parathyroid gland. In 87 patients with a functioning renal transplantation only a 11.5% of they had levels of 25(OH)D higher than 30 ng/ml and it was correlated with PTH. These remarkable findings demonstrate the importance of maintaining higher 25(OH)D levels--in addition to normal calcitriol levels--in order to avoid stimulation of the parathyroid gland in aged people. Thus, the deficiency or even "subtle deficiency" of 25(OH)D, currently neglected in the daily management of patients with chronic renal failure, may play an important role in the maintenance of hormonal and mineral homeostasis.