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Background Colon cancer patients with the same stage show diverse clinical behavior dueto tumor heterogeneity. We aimed to discover distinct classes of tumorsbased on microarray expression patterns, to analyze whether the molecularclassification correlated with the histopathological stages or otherclinical parameters and to study differences in the survival. Methods Hierarchical clustering was performed for class discovery in 88 colon tumors(stages I to IV). Pathways analysis and correlations between clinicalparameters and our classification were analyzed. Tumor subtypes werevalidated using an external set of 78 patients. A 167 gene signatureassociated to the main subtype was generated using the 3-Nearest-Neighbormethod. Coincidences with other prognostic predictors were assesed. Results Hierarchical clustering identified four robust tumor subtypes withbiologically and clinically distinct behavior. Stromal components(p BRAF mutations. Tumor subtypes werevalidated using an external set of 78 patients. A 167 gene signatureassociated to the Low-stroma-subtype distinguished low risk patients fromhigh risk patients in the external cohort (Dukes B andC:HR = 8.56(2.53-29.01); Dukes B,C andD:HR = 1.87(1.07-3.25)). Eight different reported survival genesignatures segregated our tumors into two groups the Low-stroma-subtype andthe other tumor subtypes. Conclusions We have identified novel molecular subtypes in colon cancer with distinctbiological and clinical behavior that are established from the initiation ofthe tumor. Tumor microenvironment is important for the classification andfor the malignant power of the tumor. Differential gene sets and biologicalpathways characterize each tumor subtype reflecting underlying mechanisms ofcarcinogenesis that may be used for the selection of targeted therapeuticprocedures. This classification may contribute to an improvement in themanagement of the patients with CRC and to a more comprehensiveprognosis.