Background: Elderly patients undergoing oncological surgery experience postoperative cognitive decline. The aims of this study were to examine the incidence of cognitive decline 3 months after surgery and identify potential patient-, disease- and surgery-related risk factors for postoperative cognitive decline in onco-geriatric patients. Methods: A consecutive series of elderly patients (>= 65 years) undergoing surgery for the removal of a solid tumour were included (n = 307). Cognitive performance was assessed pre-operatively and 3 months postoperatively. Postoperative decline was defined as a decline in scores of cognitive tests of >= 25% on >= 2 of 5 tests. Results: Of the patients who had completed the assessments, 117 (53%, 95% confidence interval[CI]: 47e60) had improved cognitive test scores, whereas 26 (12%, 95% CI: 7.6e16)showed cognitive decline at 3 months postoperatively. In patients aged >75 years, the incidenceof overall cognitive decline 3 months postoperatively was 18% (95% CI: 9.3e27). Inpatients with lower pre-operative MinieMental State Examination (MMSE) score (26)the incidence was 37% (95% CI: 18e57), and in patients undergoing major surgery it was18% (95% CI: 10.6e26). Of the cognitive domains, executive function was the most vulnerableto decline. Conclusion: About half of the elderly patients show improvement in postoperative cognitive performance after oncological surgery, whereas 12% show cognitive decline. Advanced age, lower pre-operative MMSE score and major surgery are risk factors for cognitive decline at 3 months postoperatively and should be taken into account in the clinical decision-making progress. Research to develop interventions to preserve quality of life should focus on this high-risk subpopulation.