Background/Aims: The increase of Nitric Oxide (NO) generated via NO synthase (NOS) contributes to pathogenesis of various disorders including cancer. Increased expression of inducible NOS (iNOS) has been reported in various cancers. However, the expression of iNOS in colorectal caner remains controversial. The aim of this study was to investigate whether iNOS is overexpressed in colorectal cancer tissue compared with normal tissue. Methods: We investigated iNOS mRNA expression in 24 human colorectal cancer tissues and normal tissues by RT-PCR and Southern blot. We also examined the expression of iNOS protein in colorectal cancer tissue and normal tissue by immunohistochemistry. Results: RT-PCR and Southern blot showed that iNOS mRNA in colorectal caner tissue was significantly increased compared with that in normal colon tissue. Immunohistochemistry showed that iNOS protein was strongly expressed in colorectal caner tissue. Conclusions: iNOS mRNA and iNOS protein are overexpressed in human colorectal cancer tissue. These results suggest that overexpression of iNOS contributes to the pathogenesis of colorectal cancer. (Korean J Gastroenterol 2001;38:247-253)