To examine the relationship between p-STAT3 and the clinicopathological parameters of colorectal adenocarcinoma (CRA), we initially conducted immunohistochemical (IHC) analyses on formalin-fixed tissues. A total of 127 invasive CRA, 20 colorectal adenomas and 20 normal mucosae were obtained. To clarify the validity of p-STAT3 as determined by the IHC analysis, quantitative real-time PCR was performed on fresh samples from 51 CRA-4 carcinomas in situ, 47 invasive CRA and on 51 normal mucosae. IHC analyses were conducted after formalin fixation from 51 CRA for comparison. The statistically significant difference of immunoreactivity for p-STAT3 between the CRA and adenoma, and between the CRA and normal mucosae was identified. Among the 174 CRA, p-STAT3 immunoreactivity significantly correlated with the T- and clinical stage. Among the 47 invasive CRA, the expression of STAT3 as determined by real-time PCR significantly correlated with tumor size, M stage and clinical stage. The overall findings of the real-time PCR analyses correlated with the findings of the IHC analyses. These findings suggest that p-STAT3 expression has an important role related to the tumorigenesis and tumor progression of CRAs. Moreover, IHC analysis is a reliable and useful modality of assessing the status of p-STAT3 expression in formalin-fixed samples.