Background: Questionable occlusal caries (QOC) can be defined as clinically suspected caries with no cavitation or radiographic evidence of occlusal caries. To the authors' knowledge, no one has quantified the prevalence of QOC, so this quantification was the authors' objective in conducting this study. Methods: A total of 82 dentist and hygienist practitioner-investigators (P-Is) from the United States and Denmark in The Dental Practice-Based Research Network (DPBRN) participated. When patients seeking treatment had at least one unrestored occlusal surface, P-Is quantified their number of unrestored occlusal surfaces and instances of QOC, if applicable. P-Is also recorded information about characteristics of patients who had QOC and had provided informed consent. The authors adjusted for patient clustering within practices. Results: Overall, 6,910 patients had at least one unrestored occlusal surface, with a total of 50,445 unrestored surfaces. Thirty-four percent of all patients and 11 percent of unrestored occlusal tooth surfaces among all patients had QOC. Patient- and surface-level QOC prevalences varied significantly according to DPBRN region (P <.001 and P =.03, respectively). The highest percentages for patient- and surface-level prevalence occurred in Florida and Georgia (42 percent and 16 percent, respectively). Conclusions: To the authors' knowledge, this is the first study in which investigators have quantified the prevalence of QOC in routine clinical practice. These results document a high prevalence overall, with wide variation in prevalence among The DPBRN's five main regions. Clinical Implications: QOC is common in routine practice and warrants further investigation regarding how best to manage it.