We investigated the expression of the cell cycle regulatory proteins cyclin D1 and p21(WAF1/CIP1) (p21) in human colorectal carcinomas using immunohistochemistry. Cyclin D1 was not detected in normal colonic epithelium; however, expression was observed in 74/126 (58.7%) of the tumour samples studied. Protein was detected in the nucleus in 22/126 (17.4%) and exclusively in the cytoplasm in 52/126 (41.3%) tumours. Nuclear expression of cyclin D1 was associated with poorly differentiated tumours (p = 0.035) and was more common in right- than in left-sided tumours (p = 0.005). Tumours displaying either, expression of cytoplasmic, (p = 0.05, HR 0.56, 95% CI 0.31-1.0) or nuclear (p = 0.021, HR 0.24, 95% CI 0.07-0.81) cyclin D1 were associated with improved patient survival compared with tumours negative for cyclin D1. p21 protein was strongly expressed mainly in the upper crypts of normal colonic epithelial cells, but in 63/126 (50%) of the tumour samples studied p21 expression was absent. Patients with tumours in which >50% of cells expressed p21 had improved survival compared to patients whose tumours were negative or had < or =50% of cells expressing p21 (p = 0.06, HR 0.33, 95% CI 0.1-1.0). We also observed a significant association between cyclin D1 subcellular localisation and p21 expression: 21/22 (95.5%) tumours expressing cyclin D1 in the nucleus also expressed p21, whereas only 17/52 (32.7%) of the tumours displaying exclusive cytoplasmic cyclin D1 staining were positive for p21 (p < 0.001). These data highlight the significance of exclusive cytoplasmic expression of cyclin D1 in colorectal cancer and lend support to recent in vitro studies suggesting that p21 protein may modulate the subcellular localisation of the cyclin D1 protein. Thus, deregulated expression of the cyclin D1 and p21 proteins are important in colorectal tumourigenesis and have implications for patient prognosis.