Herpes simplex virus 1 (HSV-1) reactivation is frequent in intensive care unit patients (20% to 50%, depending on case-mix): it begins in the throat after 3 to 5 days of hospital stay, then progress towards the respiratory tract to reach the lung after 7 to 10 days. In few cases, a HSV-1 bronchopneumonitis (defined by clinical symptom, HSV-1 in the lower respiratory tract and either specific cytopathic effect on cells collected during bronchoalveolar lavage or a HSV-1 load above 105 copies/million cells), can occur. HSV-1 reactivation is associated with mortality, but whether it is only a bystander or it has its own morbidity/mortality is unknown. Two randomized trials have shown lack of efficacy of acyclovir prophylactic or preemptive treatment, but observational studies suggest that acyclovir may be beneficial as a curative treatment. To date, we can only recommend acyclovir in patients with HSV-1 bronchopneumonitis or with HSV-1 load above 105 copies/million cells in the bronchoalveolar lavage.