Colorectal cancer (CRC) is the second most common cause of cancer related death in western countries after lung cancer in men and breast cancer in women, acts a major health problem in Iraq. To study the role of serum serotonin hormone levels as inflammatory marker in colon cancer patients at Cancer Centre of Al-Sadr General Teaching Hospital in Al-Najaf Governorate. Retrospective study was conducted in a single center, Al-Sadr general teaching hospital during the period from January to April 2022. A sample size of 83 of Colorectal Carcinoma patient included their results compared with 37 healthy persons (the total 120). Data analyzed by Mean, SD with p-value (0.05) using SPSS program. The mean age of the patients at the time of diagnosis was determined with for each status. Data analysis revealed that there is a general increase in the incidence of colorectal cancer across the years from total sample across the years, the predominant site of the tumor is colon, the frequency of CRC was higher among patients >45 years. Serotonin gives positive effects according to disease main groups in some colon cancer cases. also serotonin gives positive effects on colon cancer cases at in 8x8 follow-up study at many times. Serotonin challenge gives positive effects in study subjects (patients & control) of both sexes with its related mean± SD & p-values for each case. Serotonin challenge of 8x8 follow-up study also gives positive effects in disease main groups in many colon cancer cases. Further studies are required to explain the detailed mechanism of serotonin's effect on colon cancer.