Purpose: with an increased risk of complications. Improved preoperative risk stratification and earlier diagnosis of these complications may ameliorate postoperative recovery and improve long-term outcomes. The perioperative longitudinal study of complications and long-term outcomes (PLUTO) aims to establish a comprehensive biorepository that will facilitate research in this field. Patients undergoing elective intermediate to high-risk non-cardiac surgery are eligible for enrolment. For the first 7 postoperative days (or longer as indicated), participants will be subjected to daily bedside visits by dedicated observers, who adjudicate clinical events and perform non-invasive physiological measurements (including handheld spirometry and single-channel EEG). In addition, we will collect blood samples as well as microbiome specimens at selected time points. Primary study outcomes are the postoperative occurrence of nosocomial infections, major adverse cardiac events, pulmonary complications, acute kidney injury and delirium. Secondary outcomes include mortality as well as long-term psychopathology, cognitive dysfunction, and quality of life. PLUTO is the first perioperative biobank worldwide that includes a broad range of high-risk surgical patients, collecting prospective bedside data as well as both blood and microbiome specimens during the entire perioperative period. The data and materials collected in PLUTO will be used to develop, externally validate, and update prognostic prediction models for improved risk assessment, to test novel biomarkers for early detection of postoperative complications and to study the aetiology, attributable morbidity and mortality related to these events.