INTRODUCTION: Oral sodium phosphate (NaP) appears to be better tolerated than standard polyethylene glycol (PEG) solution in the preparation of patients for colonoscopy because of the much smaller volume to be ingested. On the contrary, safety of NaP is limited by the risk of serum electrolyte imbalance.We hypothesized that the association of half dose of both NaP and PEG preparations could improve the safety of NaP as well as the tolerance to PEG while maintaining the cleansing effectiveness. METHODS: One hundred and five patients were prospectively randomized to receive one of three preparations in a single-blind study. Group 1 received 3 liters of PEG solution, group 2, 90 ml of NaP solution and group 3 received 1,5 liters of PEG and 45 ml of NaP solutions. Endoscopists unaware of the type of preparation assessed cleanliness of the colon and patients were questioned about tolerance and side effects. Serum samples were taken for quantification of phosphate and calcium. RESULTS: Endoscopists scored NaP alone as significantly more effective than any of the two PEG-based preparations. Preparation with NaP were clasiffied as excellent or good in 91% of cases, compared with 32% and 31% for NaPPEG and PEG respectively (p<0.001). The mixed NaP-PEG preparation was easier to accomplish (easy to complete in 40% of cases vs 26% in patients prepared with PEG alone) and had fewer side effects than the PEG solution (8% of patients prepared with NaP-PEG and 24% of patients prepared with PEG suffered from moderate to severe symptoms secondary to the preparation, p<0.01). However, NaP was as safe (92% of patients had no or mild symptoms)and significantly easier to accomplish (easy to complete in 72% of patients) than the mixed NaP-PEG preparation. Hyperphosphatemia was observed in patients taking NaP alone (serum phosphate 4.98 ± 0.15 mg/dl) but it did not occur when PEG was associated (serum phosphate 4.0± 0.12 mg/dl). Hypocalcemia was never noted. CONCLUSION: Sodium phosphate as bowel cleansing agent is highly effective, easy to accomplish and well tolerated. Serum electrolyte alterations are of little clinical relevance. This side effect can be avoided by associating PEG to NaP, but both efficacy and tolerance decrease. PEGbased solutions can not be any longer considered as standard for bowel preparation for colonoscopy.