Regular administration of non-steroidal anti-inflammatory drugs (NSAIDs) may reduce the incidence of colorectal cancer by targeting cyclo-oxygenase-2 (Cox-2), a key enzyme in arachidonic acid metabolism. To evaluate the role of Cox-2 in sporadic colorectal cancer development, Cox-2 expression was investigated by immunohistochemistry in 85 adenomas, 53 carcinomas, 34 hyperplastic lesions and 104 samples of histologically normal mucosa adjacent to adenoma or carcinoma. In addition, Cox-2 mRNA expression was assessed by reverse transcription-polymerase chain reaction (RT-PCR) in six adenomas and 14 carcinomas with paired grossly normal mucosa. Immunohistochemistry for the proliferation-associated antigen Ki-67 and in situ end labelling for demonstrating apoptotic bodies were also used to analyse the associations between Cox-2 expression and proliferation and apoptosis. Cox-2 protein expression was increased in 76/85 (89โข4 per cent) adenomas and 44/53 (83โข0 per cent) carcinomas compared with normal mucosa. Cox-2 protein expression was unrelated either to the degree of dysplasia or to the size of the adenomas (p>0โข50, p>0โข10, respectively) or to differentiation, Dukes stage or lymph node metastasis of carcinomas (all p>0โข50). Interestingly, 20/34 (58โข8 per cent) hyperplastic lesions adjacent to adenomas or carcinomas displayed expression higher than in normal mucosa (18โข3 per cent) (p<0โข0001) but lower than in adenomas or carcinomas (p<10 5 , p<0โข001, respectively). There were no correlations between Cox-2 protein expression and proliferative or apoptotic index in either adenomas or carcinomas (all p>0โข25). Cox-2 mRNA expression was significantly increased in adenomas and carcinomas compared with normal mucosa (p<0โข005, p<0โข001, respectively). There were no differences between adenomas and carcinomas in either protein or mRNA levels (p>0โข25, p>0โข90, respectively). These data indicate that enhanced expression of Cox-2 occurs early during colorectal carcinogenesis and may contribute to tumour formation.