Objective: The purpose of this study was to investigate the efficacy and tolerance of an additional phosphate enema prior to non-laxative CT colonography (CTC). Methods: 71 patients (mean age 80 years, 28 male, 43 female) underwent non-laxative CTC following 4 oral doses of diluted 2% w/w barium sulphate. Patients were invited to self-administer a phosphate enema 2 h before CTC. An experienced observer graded the volume of retained stool (1 (nil) to 4 (.75% bowel circumference coated)), retained fluid ((1 (nil) to 4 (.50% circumference obscured)), retained stool tagging quality (1 (untagged) to 5 (>75% to 100%) tagged) and confidence a polyp >6 mm could be excluded (yes/no) for each of six colonic segments. Tolerance of the enema was assessed via questionnaire. Data were analysed between those using and not using the enema by Mann-Whitney and Fisher's exact test. 18/71 patients declined the enema. Results: There was no reduction in residual stool volume with enema use compared with non-use either overall (mean score 2.6 vs 2.7, p50.76) or in the left colon (mean 2.3 vs 2.4, p50.47). Overall tagging quality was no different (mean score 4.4 vs 4.3, p50.43). There was significantly more retained left colonic fluid post enema (mean score 1.9 vs 1.1, p,0.0001), and diagnostic confidence in excluding polyps was significantly reduced (exclusion not possible in 35% segments vs 21% without enema, p50.006). Of 53 patients, 30 (56%) found the enema straightforward to use, but 4 (8%) found it unpleasant. Conclusion: Phosphate enema use prior to non-laxative CTC leads to greater retained fluid, reducing diagnostic confidence, and is not recommended.