The objective of this work was to define the etiology of viral pneumopathies at the patients from reanimation section being under mechanical ventilation, making reference to viruses with respiratory tropism, and also to Chlamydia Pneumoniae and Mycoplasma pneumoniae. The subjects were 36 patients hospitalized into Service of Medical Reanimation from CHU Caen and who needed mechanical ventilation more than 48 hours. The samples from the patients were mostly nasal aspirate, 1 bronchial aspirate and 2 tracheal aspirates. The diagnosis tests were: the test of direct immunofluorescence (DIF) from the samples (for Influenza viruses A and B, Parainfluenza 1,2,3, Adenovirus and Respiratory Syncytial Virus (RSV), inoculation on the tissue culture of diploid cells MRC5, and at the appearance of cythopatic effect specific for Herpes Simplex Virus (HSV), it was made DIF for the detection of type 1 or 2, and also there were made 6 techniques of Polymerase Chain Reaction (PCR). The results of the tests were: at admission before installing the mechanical ventilation, 6 patients presented an infection with Rhinoviruses (RV), 3 with Influenza type A, 3 with HSV type 1 and 2 with Enterovirus. After a period of time from installing the mechanical ventilation, 8 patients presented an infection with HSV typel, among who 1 presented at admission an infection with RV, and 1 patient presented at 7 days from installing the mechanical ventilation an infection with RSV, and at 16 days an infection with HSV type 1. Thus, it could be concluded that in 25% from the cases of viral pneumopathies from patients being under mechanical ventilation it was an endogen reactivation of HSV type1 and only into a single case was diagnosed initially with an infection with RSV, after that it appeared also an infection with HSV typel.