Background: The mucocele of the appendix is a rare disease representing dilation of the appendix due to intraluminal mucin accumulation. It may exhibit benign or malignant features. The most severe complication of the appendiceal mucocele is development of pseudomyxoma peritonei – a life threatening situation characterized by excessive mucin accumulation in the peritoneal cavity as a result of mucocele perforation. Introduction: To describe the cases of appendiceal mucocele and mucinous neoplasms (AM and AMN) and its complications treated in our unit. Materials and methods: Review of 31 patients (22F/9 M; mean age 54.61 years) with AM, AMN and pseudomyxoma peritonei (PMP). The patients were divided into two groups: I (n=17, 54.84%) - simple mucocele and low-grade appendiceal mucinous neoplasm (LGAMN) and II (n=14, 45.16%) with high-grade appendiceal mucinous neoplasm (HGAMN). The patients from the first group have undergone interventions: isolated appendectomy (n=13) or in conjoint with other surgeries(n=4). Three patients from the second group have had primary interventions (appendicectomy, hysterectomy+ bilateral adnexectomy, bilateral adnexectomy) and repeated surgical procedures for pseudomyxoma peritonei: cytoreductive surgery with intra- and early postoperative hipertermic intraperitoneal chemotherapy (HIPEC and EPIC) (n=9), right hemicolectomy+HIPEC and EPIC (n=4), paracentesis+ EPIC (n=1). The HIPEC and EPIC (5POD) was performed with 5FU-750 mg/m2. Results: The post-operative follow-up: 71.76±12.32 months for the LGAMN group and 46.43±7.97 months for the HGAMN group. In the LGAMN group, one case of PMP developed vs 14 patients from the HGAMN group (5.88% vs 100%; p<0.0001). In the LGAMN group no deaths were recorded vs 5 patients who died in the HGAMN group (zero vs 35.71%; p=0.0118). The time range until PMP was diagnosed represented 1-168 (mean 34) months. Conclusion: The obtained data confirm that PMP does not develop only from malignant forms of mucocele, and therefore, all the patients with appendiceal mucinous lesions have to be followed-up. There is a statistically significant difference between the survival duration and occurrence of PMP, depending on the histological type of the appendiceal mucinous lesions which are more common in the HGAMN case.