Purpose: To identify discrepancies between intraoperative and histological evaluations of the appendix in acute appendicitis and to evaluate the effect on surgical outcome. Methods: Data was used from our previous multicentre, prospective, cohort study of patients with suspected acute appendicitis. Appendices were scored during intraoperative and histological evaluation as uncomplicated or complicated appendicitis. Primary outcome was percentage of concordance between intraoperative and histological evaluation. Secondary outcomes were (infectious) postoperative complications, length of hospital stay, hospital re-admission and re-intervention rate, all within 30 days of surgery. Results: A total of 1850 patients were included. In 65.7% (1215/1850) of the appendices, the intraoperative evaluation was uncomplicated and in 34.3% (635/1850), complicated appendicitis. Patients with uncomplicated appendicitis had a postoperative course with significantly less postoperative complications (7.2% vs 24.3%), a shorter length of hospital stay (2 vs 5 days) and a lower re-admission (4.2% vs 9.6%) and re-intervention rate (1.1% vs 4.3%) than intraoperative complicated appendicitis (p < 0.001). In 93.5% (1136/1215) of the intraoperative uncomplicated patients and in 46.6% (296/635) of the intraoperative complicated patients, there was an agreement with pathology (Kappa 0.45). In 23.9% (81/339) of patients with intraoperative complicated and histological uncomplicated appendicitis, a postoperative complication was observed, which was similar to the postoperative complication rate of complicated appendicitis both on intraoperative and histological evaluation (24.7% (73/296)). Conclusions: There is a moderate agreement between a surgeon and pathologist in diagnosing patients with complicated appendicitis. However, the intraoperative diagnosis of complicated appendicitis was significantly associated with postoperative complications. Routine histological evaluation should be preserved for excluding malignancies in suspect appendices.