Mucinous tumours of the ovary are characterized by mucin-secreting cells exhibiting a variable endocervical, intestinal, gastric or pancreatobiliary phenotype as ascertained by microscopy, electron microscopy, histochemistry or immunohistochemistry. The molecular mechanisms underlying the tumourigenesis process are not well understood. The mucin glycoproteins expressed by ovarian mucinous tumours have not been fully characterized, but mucins are known to be implicated in tumour progression in various epithelial neoplasms. The purpose of this study was to evaluate the expression of mucin genes (MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC6) in ovarian mucinous tumour cells, to relate MUC gene expression to the histological diagnosis, and to compare the expression patterns with those observed in normal tissues. The expression of mucin genes was evaluated by in situ hybridization in 21 mucinous tumours (11 adenomas and ten borderline tumours). Heterogeneity of expression correlated with morphological heterogeneity. Intense expression of the MUC5AC gene, suggesting a gastric surface cell phenotype, was demonstrated in 18/21 tumours (86%). Goblet cells expressing the MUC2 gene and columnar cells expressing the MUC3 gene were consistent with an intestinal phenotype, which was observed in 15 tumours (71%) including nine adenomas and six borderline tumours. Major expression of MUC4 and MUC5B consistent with an endocervical phenotype was observed in seven benign (64%) and three borderline (30%) tumours. In all, the MUC profiles suggested gastrointestinal-type cells in 13 cases (62%), gastric-type cells in five cases (24%), and intestinal-type cells in two cases (one benign, one borderline) (9%); the results were inconclusive in one borderline tumour (5%). It is concluded that gastric and, to a lesser degree, intestinal differentiation are early and almost constant events in ovarian mucinous tumourigenesis.