To investigate early onset pneumonia in a neurosurgical intensive care unit, we studied a cohort of patients over a 13-month period and compared neurotrauma (T) with non-neurotrauma (NT) patients. Data were abstracted from the infection surveillance database. Five hundred and sixty-five adults were hospitalized in the neurosurgical intensive care unit. 57.9% had trauma and 129 patients developed 152 episodes of pneumonia. Incidence rates, restricted to the 129 first episodes of pneumonia, were 20.1 versus 15.7/1000 patient days and 34.2 versus 27.9/1000 ventilation days, in the T and NT groups respectively. In both groups, the distribution of risk stratified by hospital days was bimodal, being highest during the first three days. However, the risk was higher for T patients (at day 3, 20/1000 ventilation days versus 10.2/1000 ventilation days). The daily risk peaked again at days 5 and 6, and thereafter remained low. Pneumonia occurring within the first three days, or early onset pneumonia (EOP), was associated with trauma (P = 0.036) and, in the NT group only, with a Glasgow coma scale score lower than 9 (P = 0.062). EOP was caused by Staphylococcus aureus (33%), Haemophilus spp. (23%), other Gram-positive cocci (22%), and other Gram-negative bacilli (GNB) (19%); whereas after the third day GNB other than Haemophilus spp. accounted for 45.4% of isolates (P = 0.11). This large series confirms the high incidence of EOP in neurosurgical intensive care units, particularly among trauma patients, in relation to risk factors different from those seen in other intensive care patients. Further studies are needed to elaborate specific preventive measures during early care.