Rheumatoid arthritis (RA), is an autoimmune disorder of unknown aetiology characterised by synovial proliferation (synovitis), cartilage degradation, bone erosion and joint swelling. RA affects 1% of the adult population and exhibits a chronic fluctuating course which may result in progressive cartilage destruction, joint deformity and disability. The earliest abnormalities in RA consist of synovitis, soft tissue swelling, and osteoporosis. In later stages, the inflamed synovial tissue (‘pannus’) extends across the cartilage surface, leading to massive chondral and small bone erosions. The chronic inflammation is often accompanied by joint swelling and oedema. Previous reports have suggested that human chondrocytes and synoviocytes express aquaporin 1 (AQP1) water channels (Mobasheri et al., 2002; Mobasheri and Marples, 2004) and that AQP1 is upregulated in RA cartilage (Trujillo et al., 2004). In the present study, we have used immunohistochemistry to investigate the expression of AQP1 water channels in human synovitis Tissue MicroArrays (TMAs) and to determine if AQP1 expression is altered in synovitis. Immunohistochemical staining of custom designed “normal” and “synovitis” TMAs revealed that AQP1 is expressed in synovial micro-vessels and synoviocytes from normal joints (n = 20 normal subjects). AQP1 was significantly upregulated in synovium derived from RA (n = 10) and psoriatic arthritis (n = 8) patients. These results indicate a potential role for AQP1 in the joint swelling associated with synovial inflammation. It remains to be determined whether other aquaporins are involved in joint swelling in synovitis.