Redox control has emerged as a fundamental biological control mechanism. One of the major redox control systems is the thioredoxin system comprised of thioredoxin (TRX) and thioredoxin reductase (TR). Together they form a powerful system involved in many central intracellular and extracellular processes including cell proliferation, the redox regulation of gene expression and signal transduction, protection against oxidative stress, anti-apoptotic functions, growth factor and co-cytokine effects, and regulation of the redox state of the extracellular environment. Over recent years this system has increasingly been linked to the development and expression of cancer phenotypes. In this report immunocytochemical approaches have been used to simultaneously determine the expression and localisation of both TRX and TR in primary human cancers, including breast cancer, thyroid, prostate and colorectal carcinoma, and malignant melanoma. In aggressive invasive mammary carcinomas and advanced malignant melanomas, thioredoxin was highly over-expressed compared to tumours of lesser aggressive nature. TRX expression was found in both nuclear and cytoplasmic location in the neoplastic cells. Furthermore, increased levels of TRX positively correlate with thioredoxin reductase (TR) expression and localisation. These results, which are the first immunocytochemical studies on the in vivo expression and localisation of TRX and TR in melanomas, thyroid, prostate and colorectal carcinomas and the first reports of TR expression in breast carcinomas, significantly extend the range of human cancers for which such data is available. Overall the results support the conclusion that aggressive tumours greatly over-express both TRX and TR. Such tumours have a high proliferation capacity, a low apoptosis rate and an elevated metastatic potential strongly implicating the involvement of the TRX system in the processes of oncogenesis and tumourogenesis and confirming its potential as a target for anticancer therapy for a wide range of human tumours.