Previous reviews of bowel preparation for colonoscopy have given contradictory answers. To provide a definitive insight, using PRISMA-compliant methodology. A comprehensive literature review identified randomised controlled trials comparing bowel preparation regimens. Data for quality of bowel preparation were pooled in multiple meta-analyses exploring a range of inclusion criteria. A total of 104 qualifying studies were identified, the majority of which involved comparisons of sodium phosphate (NaP) or polyethylene glycol (PEG). There was no significant difference demonstrated between NaP and PEG overall (OR = 0.82; 95% CI = 0.56-1.21; P = 0.36). Cumulative meta-analysis demonstrated that this conclusion has been qualitatively similar since the mid 1990s, with little quantitative change for the past 10 years. Amongst studies with previous day dosing in both study arms there was a significant advantage in favour of PEG (OR = 1.78; 95% CI = 1.13-2.81; P = 0.006). Studies focussing on results in the proximal colon also favoured PEG (OR = 2.36; 95% CI = 1.16-4.77; P = 0.012). PEG was also significantly more effective than non-NaP bowel preparation regimens (OR = 2.02; 95% CI = 1.08-3.78; P = 0.03). Other comparisons showed no significant difference between regimens. Although there is no compelling evidence favouring either of the two most commonly used bowel preparation regimens, this may reflect shortcomings in study design. Where studies have ensured comparable dosage, or the clinically relevant outcome of proximal bowel clearance is considered, PEG-based regimens offer the most effective option. © 2011 Blackwell Publishing Ltd.