Objective: To describe epidemiology and age-related mortality in critically ill older adults with intraabdominal infection.Methods: A secondary analysis was undertaken of a prospective, multi-national, observational study (Abdominal Sepsis Study, ClinicalTrials.gov #NCT03270345) including patients with intra-abdominal infection from 309 intensive care units (ICUs) in 42 countries between January and December 2016. Mortality was considered as ICU mortality, with a minimum of 28 days of observation when patients were discharged earlier. Relationships with mortality were assessed by logistic regression analysis.Results: The cohort included 2337 patients. Four age groups were defined: middle-aged patients [reference category; 40-59 years; n = 659 (28.2%)], young-old patients [60-69 years; n = 622 (26.6%)], middle old patients [70-79 years; n = 667 (28.5%)] and very old patients [ >80 years; n = 389 (16.6%)]. Secondary peritonitis was the predominant infection (68.7%) and was equally prevalent across age groups. Mortality increased with age: 20.9% in middle-aged patients, 30.5% in young-old patients, 31.2% in middle-old patients, and 44.7% in very old patients ( P < 0.001). Compared with middle-aged patients, young-old age [odds ratio (OR) 1.62, 95% confidence interval (CI) 1.21-2.17], middle-old age (OR 1.80, 95% CI 1.35-2.41) and very old age (OR 3.69, 95% CI 2.66-5.12) were independently associated with mortality. Other independent risk factors for mortality included late-onset hospital-acquired intra-abdominal infection, diffuse peritonitis, sepsis/septic shock, source control failure, liver disease, congestive heart failure, diabetes and malnutrition.Conclusions: For ICU patients with intra-abdominal infection, age > 60 years was associated with mortality; patients aged >80 years had the worst prognosis. Comorbidities and overall disease severity further compromised survival. As all of these factors are non-modifiable, it remains unclear how to improve outcomes.(c) 2022 Elsevier Ltd and International Society of Antimicrobial Chemotherapy. All rights reserved.