Aquaporin 1 (AQP1) water channels are membrane proteins that control the permeability of endothelial and epithelial barriers by facilitating water movement across cell membranes. AQP1 is present in microvessel endothelia of many normal human tissues and is highly expressed in active water transporting epithelia such as the choroid plexus and the kidney. The potential importance of AQP1 in oncology and carcinogenesis has only recently been recognized. It has been suggested that AQP1 may be responsible for the high vascular permeability and interstitial fluid pressure in tumours of the brain, colon, breast and pancreas (Endo et al., 1999). AQP1 may also play a role in tumour angiogenesis and may be involved in development of effusions or oedema fluid (Vacca et al., 2001). The aim of the present study was to use immunohistochemistry to compare the distribution and relative abundance of AQP1 on National Cancer Institute TARP (Tissue Array Research Program) human multiple tumour Tissue MicroArrays (TMAs) with normal tissues represented on the National Institutes of Health CHTN (Cooperative Human Tissue Network) TMAs. Immunohistochemistry and semi-quantitative histomorphometric analysis were used to compare the distribution of AQP1 in tumours of the prostate, colon, lung, breast and ovary represented on TARP TMAs with their normal counterparts on CHTN TMAs. AQP1 was expressed in capillary endothelia of all normal tissues. In most tumours AQP1 was confined to endothelial barriers. AQP1 expression was marginally higher in microvascular structures in prostate and ovarian tumours and was higher in a small number of advanced mammary and colorectal carcinomas where AQP1 immunoreactivity was also seen in some neoplastic tumour cells. We therefore conclude that AQP1 is heterogeneously expressed in different human tumours and is rarely present in neoplastic cells. Increased AQP1 expression in some human adenocarcinomas may be a consequence of angiogenesis and important for the formation or clearance of tumour oedema. These preliminary data will form the basis of future studies to investigate the significance of AQP1 in solid tumours of humans and veterinary species.