Epidemiological studies have shown that the risk of developing oesophageal adenocarcinoma (OA) is inversely correlated to consumption of fruits and vegetables. The effects of proanthocyanidins (PA), which are the most abundant subclass of flavonoids in the diet, on OA cells in vitro were investigated. Three apple-derived PA fractions were characterized for their flavan-3-01 composition, and their effects on OA cells were assessed. All these PA fractions caused a reduction in cell viability, in association with an arrest of the cell cycle in the 00/01 phase and an induction of apoptosis. Also, the PA-induced reductions in cell viability and increases in apoptosis were concentration-, DP- and time-dependent. Indeed, these biological effects were observed after relatively short treatments with PA (cell cycle arrest and induction of apoptosis observed after 1 and 2 hours, respectively). These cellular effects were accompanied by a rapid, strong and time-dependent up-regulation of p21Cipl/WAFI expression (which was induced in a p53-independent manner), suggesting that p21 may be the molecular mediator of the observed effects. However, blocking the PA-induced up-regulation of p21 expression with siRNA did not alter PA-mediated changes in apoptosis and cell cycle, demonstrating that p21 is not responsible for the PA-induced effects. Therefore other as yet unidentified molecular mechanisms mediate the cell response to PA. Further investigation of the alterations in gene expression following PA exposure was performed using microarray technique. These allowed the identification of signalling pathways affected by PA exposure (including the MAPK, TOF-P and Wnt signalling pathways), and of possible biomarkers of cell responses to PA (e.g. DUSPl and DUSP10).