The caudal-related homeobox transcription factor CDX2 regulates the differentiation of intestinal epithelial cells. Recent studies have suggested that CDX-2 immunoreactivity is strictly confined to benign and malignant intestinal epithelium. In the present study, we evaluated the prevalence of CDX2 immunoreactivity in a series of benign, borderline and malignant primary ovarian mucinous neoplasms. We tested 62 mucinous tumours of the ovary, including 28 benign cystadenomas, 18 borderline tumours, 16 adenocarcinomas, 35 serous and endometrioid ovarian lesions and 10 ovarian metastases of colonic adenocarcinoma. Overall, the CDX2 prevalence in primary mucinous tumours was 79%, including 20 of 28 (71.5%) cystadenomas, 14 of 18 (77.7%) borderline tumours and 15 of 16 (93.5%) adenocarcinomas. Immunoreactivity usually correlated with intestinal differentiation of tumour cells, although wide heterogeneity in the distribution of immunolabelled cells was noted. No immunoreactivity was observed in serous lesions; whereas, 1 of 13 (7.7%) endometrioid adenocarcinomas and all of the 10 metastatic colonic adenocarcinomas were immunostained. These results indicate that CDX2 is detectable in the majority of benign, borderline and malignant ovarian mucinous tumours and, therefore, makes this marker unsuitable when distinguishing primary from metastatic ovarian mucinous adenocarcinomas. However, CDX2 immunoreactivity could be useful in the distinction between endocervical and intestinal-type mucinous tumours of the ovary, which may have clinical relevance.