The incidence of heterotopic ossification (HO) among patients with traumatic brain injury (TBI) varies in the literature from 11 to 73.3%. The aim of this study was to determine the incidence of HO among patients with very severe TBI treated in a new established intensive rehabilitation Brain Injury Unit and to list some of the risk-predicting features. The study comprised an approximately complete, consecutive series of 114 adult patients from a well-defined geographical area, and with a posttraumatic amnesia period of at least 28 days, i.e. very severe TBI. Demographic and functional data as well as data about trauma severity and hospital stay of these patients have been registered prospectively in a database (Danish National Head Injury database) at the Brain Injury Unit where the sub acute rehabilitation took place. The present study was based retrospectively on this database, combined with X-rays obtained for symptoms of HO and/or as fracture control. Clinically significant HO was found in 7.9% of the patients. Logistic regression showed an independent significant positive correlation between HO, the female gender and a high Injury Severity Score. The low incidence of HO might be explained by the application of early mobilisation and physiotherapy of the patients. The higher incidence of HO among women speaks for humoural and hormone factors initiating bone formation outside the bones.