<b>Background:</b> <i>Clostridium difficile </i>associated diarrhea (CDAD) is the main cause of nosocomial diarrhea in the world. &nbsp; Recently there was an outbreak of CDAD in the San Juan de Dios Hospital, San José -Costa Rica; a tertiary care center with 700 beds. This study analyses the epidemiological, clinical and microbiological characteristics of the inpatients with CDAD treated from November 2008 to June 2009. &nbsp; <b>Population and methods:</b> A CDAD case was defined as a patient with diarrhea and with positive ELISA for <i>C. difficile </i>A toxin in feces. An analysis of the annual incidence of CDAD from 2004 to 2008, and the monthly incidence in 2009 at HSJD was made, as well as a retrospective and observational study of 112 medical records for patients diagnosed with CDAD treated at this hospital, from November 15, 2008 to June 15, 2009. The analysis of the data was made using descriptive statistics and measures of association. &nbsp; <b>Results:</b> The incidence of CDAD increased significantly since the end of 2008 and reached its maximum peak in April 2009, when sanitary measures were implemented. They reduced by 75% the number of patients with CDAD in 8 months. Of the 112 medical records reviewed, 63 (56%) were men. The mean age was 65.33 years, 103 (92%) patients developed the disease while hospitalized; the mean period of hospitalization was 18.6 days. Only 9% did not suffer from any comorbidity. The most frequent comorbidities were: hypertension and type 2 diabetes, with 57.5% and 39.8%, respectively, and neuropsychiatric disease with 29.2%. A 96% (107 patients) had received three or more antibiotics before the onset of diarrhea. The mean duration of antibiotic therapy was 32 days per patient. In average, the duration of diarrhea was 10.2 days (1-90 days). Most patients were treated with metronidazole or vancomycin. The mortality directly associated to CDAD was 7%. &nbsp; <b>Discussion:</b> An outbreak of CDAD in a national hospital with 700 beds is described. It was observed more frequently in elderly (&gt;65 years) with long hospital stays, with multiple comorbidities and who had received multiple antibiotics for prolonged periods; mainly cefotaxime, fluoroquinolones or clindamycin. After sanitary and medical measures were implemented, the impact was reduced in 75% over 8 months

Diarrhea; <i>Clostridium difficile</i>; outbreak; nosocomial infection