Clinical practice shows that patients with constipation are more likely to have a poor bowel preparation for colonoscopy. Consequences of poor bowel preparation include missed lesions, increased complication rates, and higher costs. Research on preparing constipated patients for colonoscopy with Food and Drug Administration-approved medications is scarce. This study included 372 patients and compared the effectiveness of high-volume (4 L) polyethylene glycol solution (GoLYTELY) with low-volume (2 L) polyethylene glycol solution with ascorbate (MoviPrep) in constipated and nonconstipated adults. Patients were considered constipated if constipation was recently documented in their medical history, they self-reported constipation, and/or they met Rome III criteria for constipation. Quality of bowel preparation was rated with the Aronchick scale. Of the 372 patients enrolled, 65 (17.5%) were constipated. Among those with constipation, 26.1% (n = 17) had a bowel preparation rated fair or poor. There was no statistically significant difference in quality of bowel preparation between GoLYTELY and MoviPrep in chronically constipated adults (p = .27). This study concluded that patients with constipation were more likely than nonconstipated patients to have poor bowel preparation, and there was no statistically significant difference between GoLYTELY and MoviPrep. Further study is needed to determine an adequate preparation regimen.