INTRODUCTION: Patients admitted to intensive care following surgery for faecal peritonitis present particular challenges in terms of clinical management and risk assessment. Collaborating surgical and intensive care teams need shared perspectives on prognosis. We aimed to determine the relationship between dynamic assessment of trends in selected variables and outcomes. METHODS: We analysed trends in physiological and laboratory variables during the first week of ICU stay in 977 patients from 102 centres across 16 European countries. The primary outcome was 6-month mortality. Secondary end-points were ICU, hospital and 28 day mortality. For each trend, Cox proportional hazards (PH) regression analyses, adjusted for age and gender, were performed for each endpoint. Trends remaining significant after Bonferroni correction for multiple testing were entered into a multivariate Cox PH model to determine independent associations with mortality. RESULTS: Trends over the first 7 days ICU stay independently associated with 6 month mortality were worsening thrombocytopaenia (mortality HR = 1.02, 95% CI 1.01-1.03, p < 0.001) and renal function (total daily urine output HR = 1.02, 95%CI 1.01-1.03, p < 0.001; renal SOFA sub-score HR = 0.87, 95%CI 0.75-0.99, p = 0.047), maximum bilirubin level (HR = 0.99, 95%CI 0.99-0.99, p = 0.02) and GCS SOFA sub-score (HR = 0.81, 95%CI 0.68-0.98, p = 0.028). Changes in renal function (total daily urine output and renal component of the SOFA score), GCS component of the SOFA score, total SOFA and worsening thrombocytopaenia were also independently associated with secondary outcomes (ICU, hospital and 28 day mortality). We detected the same pattern when analysing trends on days 2, 3 and 5. Dynamic trends in all other measured laboratory and physiological variables and in radiological findings failed to be retained as independently associated with outcome on multivariate analysis. Furthermore, changes in respiratory support, renal replacement therapy and inotrope and/or vasopressor requirements were not independently associated with any of the primary or secondary outcomes. CONCLUSIONS: Only deterioration in renal function, thrombocytopaenia and SOFA score over the first 2, 3, 5 and 7 days ICU stay were consistently associated with mortality at all endpoints. These findings may help to inform clinical decision making in patients with this common cause of critical illness.