Introduction Despite the recommendations to vaccinate annually against the flu to all the elderly and people with various chronic diseases, these recommendations are not comon to fulfill. In this case-control study performed in the service of Internal Medicine of the Hospital of Cáceres we have analyzed the degree of utilization of the flu vaccination in the season 2000/2001 in patients over 65 years of age with high-risk chronic diseases, as well as the effectiveness of this vaccination to avoid the hospitalization induced by cardiorespiratory decompensation, in order to reduce the number of consultations to the primary care physician and in order to reduce the issue of days of hospitalization. Patients of method 227 patients over 65 years of age with chronic cardiorespiratory disease, diabetes, chronic renal insufficiency, hepatopathy, previous pneumonia, or other causes of immunosuppression were studied. Of them, 116 were admitted because of cadiac or respiratory decompensation (cases); the control group was made up of 99 patients who went to outpatient consultations, with clinical manifestations similar to the cases and they were not hospitalized during the year of study. All the participants filled a questionnaire that included demographic characteristics and data about the underlying disease; the participants were grouped according to the number of underlying disease (one, two or more), according to if had received correctly the flu vaccination, according to the number of consultations to its Family doctor during the year of the study because of respiratory infections, according to the hospitalizations during the previous year and – in the patients that were hospitalized – acording to the number of days of the hospitalization. Results The average age was 71 years and 63% patients had been hospitalized the previous year. The percentage of vaccinated was of 60% and the vaccination was applied most frequently to the patients older than 75 years (p < 0.001), with EPOC (p < 0.005), and with cardiac insufficiency (p < 0.01), as well as to the patients with 2 or more risk factors (p < 0.001). Differences were not observed among the cases and the controls with regard to the age, to the incidence of ICC, to the incidence of DM, nor to the presence of 2 or more risk factors; however, the patients who were hospitalized presented a greater incidence of chronic obstructive pulmonary disease (COPD) (OR: 3.6; IC: 2.01-6.45) and of previous pneumonía (OR: 5.24: CI: 2.4-11-14): The factors most influencing the possibility of hospitalization were: EPOC (OR: 3.67; CI: 1.90-7.13); previous pneumonía (OR: 3.88: CI: 1.69-8.95). The estimate of hospitalizations avoided by the vaccination was of 59 (OR: 0.41; IC: 0.22-0.79). The vaccination did not decrease the number of consultations to the physician nor the days of the hospitalization. Conclusions The flu vaccination looks underused in patients over 65 years of age with multiple diseases in the hospital environment. The vaccination seems to be effective in order to diminish the number of hospital admissions because of cardiorespiratory decompensation, even in non-epidemic seasons. We should insist on the use of the flu vaccination in these high-risk patients.