It is known that the signal transducer and activator of transcription 3 (STAT3) is a key signaling molecule implicated in the regulation of growth and malignant transformation. Constitutive activation of STAT3 has been observed in a number of tumour-derived cell lines, as well as in a wide variety of human malignancies. The present study was conducted to examine p-STAT3 (activated form of STAT3) expression and its association with clinicopathological factors and prognosis in human colorectal adenocarcinomas. Expression of p-STAT3 was immunohistochemically examined in 108 cases of colorectal adenocarcinoma tissue obtained at surgery. and was found in 57.4% of tumours (62 of 108). p-STAT3 immunoreactivity significantly correlated with the depth grading of tumour invasion (p<0.001), lymphatic invasion (p<0.05), Dukes' classification (p<0.05), stage (p<0.001) and prognosis after operation (p<0.001). Expression of p-STAT3 was a marker of poor prognosis in overall survival (p<0.01). Expression of p-STAT3 was detected by Western blot analysis in three colon carcinoma tissue samples obtained at surgery. To our knowledge, this is the first study on the poor prognosis of p-STAT3 in human colorectal adenocarcinomas. These findings suggest that expression of p-STAT3 is an important factor related to tumour invasion and poor prognosis of human colorectal adenocarcinoma.