The CIAO Study is a multicenter observational study currently underway in 66 European medical institutions over the course of a six-month study period (January-June 2012). This preliminary report overviews the findings of the first half of the study, which includes all data from the first three months of the six-month study period. Patients with either community-acquired or healthcare-associated complicated intra-abdominal infections (IAIs) were included in the study. 912 patients with a mean age of 54.4 years (range 4-98) were enrolled in the study during the first three-month period. 47.7% of the patients were women and 52.3% were men. Among these patients, 83.3% were affected by community-acquired IAIs while the remaining 16.7% presented with healthcare-associated infections. Intraperitoneal specimens were collected from 64.2% of the enrolled patients, and from these samples, 825 microorganisms were collectively identified. The overall mortality rate was 6.4% (58/912). According to univariate statistical analysis of the data, critical clinical condition of the patient upon hospital admission (defined by severe sepsis and septic shock) as well as healthcare-associated infections, non-appendicular origin, generalized peritonitis, and serious comorbidities such as malignancy and severe cardiovascular disease were all significant risk factors for patient mortality. White Blood Cell counts (WBCs) greater than 12,000 or less than 4,000 and core body temperatures exceeding 38 degrees C or less than 36 degrees C by the third post-operative day were statistically significant indicators of patient mortality.