Background: Laxative pretreatment decreases the volume of polyethylene glycol electrolyte lavage solution (PEG-ELS) required for colonoscopy without compromising preparation quality. We compared the use of 4 L of PEG-ELS with the use of 2 L plus a laxative. Methods: One hundred fifty consecutive patients (148 men) undergoing outpatient colonoscopy were randomly selected for one of three preparations (Prep 1: 4 L PEG-ELS; Prep 2: 2 L PEG-ELS plus 296 mL magnesium citrate 1 hour prior; Prep 3: 2 L PEG-ELS plus bisacodyl 20 mg). Endoscopists were blinded as to the type of preparation. Results: Colonoscopy times were 37, 33, and 29.5 minutes (p = 0.02). Satisfaction scores (0 to 11) during preparation were 2.75, 1.84, and 2.54 (p = 0.05). Preparation times were 519, 397, and 379 minutes (p < 0.001). Preparation satisfaction scores (0 to 10) were 6.2, 7.7, and 7.4 (p < 0.001). Endoscopists' scores of preparation quality (1 to 10) were 7.3, 7.8, and 8.1 (p = 0.03). Volumes of liquid stool aspirated were 181, 103, and 90 mL (p < 0.001). Twenty-four patients receiving Prep 2 and 16 receiving Prep 3 had previous colonoscopy using full volume PEG-ELS; 88% who received Prep 2 and 56% who received Prep 3 preferred the newer preparation (p = 0.006). Conclusions: Two liters of PEG-ELS plus laxative improved preparation quality and patient satisfaction and reduced preparation time. Magnesium citrate pretreatment had fewer symptoms and was preferred to bisacodyl. PEG-ELS in 2 L quantities could reduce costs, and consideration should be given to making it available commercially. (Gastrointest Endosc 1998;47:167-71)