Interstitial pneumonia is the most common disease caused by infection from cytomegalovirus (CMV) in immunodepressed patients, whereas it is a rare complication in immunocompetent patients. With reference to the second group of patients, little literature has been produced as for the therapy to choose when symptoms are serious. We report the case of immunocompetent adolescent with CMV pneumonitis who responded dramatically to therapy with ganciclovir. For a week B. M., a 15-year-old girl, has been showing fever, cough and boring pain at her left thoracic base. When hospitalized, the girl was suffering and dyspneic, cardio-thoracic conditions were bad. Spleen and liver were palpable two fingers far form costal arch. Hematochemical tests showed an increase in phlogosis and transaminase value. Thoracic X-ray was negative, as well as cultures. Among the serological tests high response of anti-CMV IgM was remarkable. Virological blood test confirmed active CMV infection. On the fifth day, a thoracic radiography showed widespread interstitial infiltrates. Treatment with ganciclovir--i.v. 6 mg/kg/day, twice a day for twelve days--has been then adopted. After two day treatment, the girls was apyretic and eupneic. After ten day treatment, thoracic radiography was negative and a great decrease in CMV antigenic response was given by blood tests. No side effect were observed. According to our experience we can say that treatment with ganciclovir may positively shorten the course of pneumonia caused by CMV in immunocompetent patients.