AIM: To compare the efficacy and acceptance of senna tablet and sodium phosphate solution for bowelpreparation before colonoscopy.METHODS: One hundred and thirty four patients, who needed elective colonoscopy, were randomly allocated to take 180 mg senna tablet or 95 mL sodium phosphatesolution on the day before colonoscopy. The efficacies of both laxatives were compared using the mean difference of colon-cleanliness score of the rectum,sigmoid segments, descending colon, transverse colon and cecum. The scores were rated by two observers who were blinded to the laxatives administered. The higher score means that the colon is cleaner. The efficacy of both laxatives were equivalent if the 95% confidence interval of the mean difference of the score of colon lie within -1 to +1.RESULTS: On intention-to-treat analysis, the mean cleanliness scores in the four segments of colon except the cecum were higher in the sodium phosphate group than those in senna group (7.9 ± 1.7 vs 8.3 ± 1.5, 8.0 ±1.8 vs 8.5 ± 1.4, 7.9 ± 2.0 vs 8.5 ± 1.3, 7.9 ± 2.0 vs 8.2± 1.4 and 7.2 ± 1.7 vs 6.9 ± 1.4, respectively). The 95%confidence intervals (95% CI) of mean difference in each segment of colon were not found to lie within 1 point which indicated that their efficacies were not equivalent. The taste of senna was better than sodium phosphate solution. Also, senna had fewer side effects.CONCLUSION: The efficacy of senna is not equivalent to sodium phosphate solution in bowel preparation for colonoscopy, but senna may be considered an alternative laxative.