The circadian system is involved in the control of cell proliferation and apoptosis. The aim of this study was to analyze expression of the human per2 gene in patients who underwent surgery for colorectal carcinoma. The study included 25 patients of both genders. Patients were exposed to light from 6:00 a.m. until 9:00 p.m. according to standard hospital practice. Tissue samples were taken from the tumor as well as from the proximal and distal areas of the resected colon at the time of surgery. Surgery was performed during the morning hours. Expression of per2 mRNA was measured by real-time PCR. There was a significant negative correlation between per2 gene expression and tumor staging. Expression of per2 mRNA did not correlate with whether the tumor was localized in the colon or rectum. In comparison with ectomized tissue without malignancy from patients with colorectal carcinoma, our data demonstrate per2 mRNA deregulation in tumor tissue, and suggest a way in which the circadian system can influence tumorigenesis.