OBJECTIVE: Dentists in the National Dental Practice-Based Research Network are offered online and mail options for most questionnaire studies. We sought to quantify differences a) in characteristics of dentists who completed a questionnaire online as compared with those who completed a mail option offered to online nonresponders and b) in prevalence estimates for certain practice characteristics. METHODS: Invitation letters to participants provided an identification number and log-in code with which to complete the online survey. Nonrespondents received a reminder letter after the fourth week, and after an additional 4-week period, a final reminder was sent, along with a paper questionnaire version, allowing completion online or by paper. RESULTS: Of 632 US dentists who completed the survey, 84 (13 percent) used the paper version. Completion by paper was more common among males, older dentists, and those in general practice (P < 0.05). The proportions of dentists who used electronic dental records, who consistently used a rubber dam when performing root canals, and who either worked with or employed expanded-function auxiliaries were lower among dentists who completed the survey using the paper-mail version than among those who completed it online; these differences remained significant in models adjusted for gender, age, and practice type. CONCLUSION: Even in an era of increasingly electronic communication by dentists, not including a paper option when conducting surveys can result in overestimation of the prevalence of key dental practice characteristics.