The bone is often considered as a slow exchanger and bone sampling from the iliac crest, by needle-biopsy, is a commonly used and atraumatic technique; therefore the significance of the evolution of CO2 stores has been investigated in iliac crest in respiratory failure. Samples are taken before ventilation and on the first, second, and in some cases third days after ventilation. The results show that ageing is to be considered, since the CO2/Ca ratio varies according to the age group (0.84 at 30 years and 1.35 at 60 years). Patients with respiratory failure may be distributed into two main categories: hypoxic patients without hypercapnia, and patients with predominant hypercapnia. Hypercapnic patients may be differentiated from the hypoxic ones, both before assisted ventilation and on account of the dynamic phenomena that come into play during respiratory support in this group of patients. If the equilibrium state (at least five days with rigorously steady gazometry), before assisted ventilation, remains unknown, it is not possible to distribute the patients in one of these two groups. The results' significance is discussed, as well as the possible therapeutic applications.