The membrane-bound MUC1 mucin is expressed in normal mucosas and the aberrant expression of its under-glycosylated forms has been reported in carcinomas from different sites. Several studies have provided conflicting evidence regarding the relationship between MUC1 expression and outcome in cancer patients. In this study, we investigated the immunohistochemical expression of MUC1 epitopes, using 2 monoclonal antibodies (MAbs): HMFG1, which reacts with the fully glycosylated MUC1, was studied in 73 gastric carcinomas; and SM3, which recognises an under-glycosylated form of MUC1, was studied in 180 cases. HMFG1 stained the antrum foveolar cells and the body glands of normal gastric mucosa, whereas SM3 reactivity was restricted to the perinuclear region of some foveolar cells. Type I intestinal metaplasia exhibited down-regulation of MUC1 expression using both MAbs. Every gastric carcinoma was stained with HMFG1 and 80% with SM3. High levels of expression of HMFG1 were associated with lymphatic invasion, nodal metastatization, and advanced pTNM staging. The expression of SM3 was associated with the histologic (solid) type of carcinoma, expanding growth pattern, wall penetration, lymphatic invasion and age of the patients. Despite a trend for a poor outcome in patients with tumours (over)expressing MUC1 mucin, the survival of the patients evaluated by univariate and multivariate analysis was not significantly associated with the levels of expression of HMFG1 or with the expression of the SM3 epitope. We conclude that (a) MUC1 expression, namely of the SM3 cancer-associated epitope, is significantly associated with several aspects of gastric cancer development and progression; and (b) MUC1 expression should not be used as a prognostic marker in patients with gastric carcinoma.