Critical illness has been recognized to acutely influence bone metabolism and, consequently, bone mineral density. The main purpose of this study was to describe bone metabolism changes in adult survivors of critical illness in the attempt to correlate changes with severity scores. It is an open, prospective, observational, monocentric study on patients admitted to the ICU was conducted, evaluating bone metabolism at baseline (within 72 hours of ICU admission), 6 months, and 12 months. Fifty-nine patients admitted to the ICU (63% males), mean age 58 ± 16 years, were enrolled. Of these, 20 patients (34%) completed the one-year follow up. At baseline, bone resorption showed an increase, which was maintained at 6 months, with normalization at 12 months. Patients showed, in a majority of cases, hypovitaminosis D with hyperparathyroidism at baseline with subsequent normalization. A trend towards a correlation was described between severity scores and serum 25(OH) vitamin D and bone turnover marker levels. These results contribute to the confirmation of a positive association between critical illness requiring ICU and bone metabolism changes. This study poses the bases for further studies to evaluate bone health in ICU patients.