A prospective study was conducted to evaluate pituitary-gonadal function and correlated parameters in 21 adult males with severe traumatic brain injury during acute inpatient rehabilitation. Serum concentrations of testosterone, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were measured within 1 week after the patient was transferred to the rehabilitation unit. Fourteen of 21 patients (67%) had abnormally low testosterone levels. One of 21 patients had a subnormal FSH level and one had a supranormal level. Three of 21 patients had subnormal LH levels and two had supranormal levels. There was no correlation between the severity of brain injury and the levels of testosterone, FSH or LH. The presence of increased intracranial pressure, hypoxia, skull fracture or abnormal CT findings had no significant influence on the levels of testosterone, FSH or LH. The high incidence of hypotestosteronaemia in survivors of severe traumatic brain injury is seemingly more related to accompanying physiological stressors rather than structural or neurochemical disruption of the hypothalamic-pituitary-gonadal axis. Early identification is important relative to the potential neuromedical and rehabilitative consequences of prolonged hypotestosteronaemia in this patient population.