We investigated the prognostic significance of microsatellite instability (MI) in 50 consecutive patients with sporadic mucinous colorectal cancer who had undergone only surgery. We evaluated MI and the pathological features with a possible prognostic value for each tumor, and the effect of the examined parameters on patients' outcome was statistically analyzed (univariate and multivariate analysis). All patients were followed-up for a minimum of 72 months or until death; in evaluating survival, only deaths of colorectal cancer were considered. DNA extracted from tumor sections and the corresponding normal tissue was analyzed by polymerase chain reaction at six microsatellite loci: D2S123, D3S1611, D3S49, D5S107, BAT26, BAT40. Alterations at two or more loci were detected in 36% of cases (MI+ tumors). MI+ and MI- cancers differed significantly in the pattern of growth, and most MI+ tumors showed an expanding type of growth (72.2%, P = .005). At univariate analysis, improved survival rate was significantly associated with MI, as well as with the following parameters: expanding cancer growth, Dukes stage, and absence of venous invasion. Nevertheless, at multivariate analysis, only the pattern of cancer growth and Dukes stage were independent prognostic factors, whereas the effect on survival of MI and venous invasion was found to be negligible. In our study, MI+ and MI- cancers differ only on the pattern of growth; therefore, our data suggest that the better survival rate in mucinous cancers with genomic instability is strictly related to their less aggressive type of growth.