Objectives: Questionable occlusal caries (QOC) can be defined as clinically-suspected occlusal caries with no cavitation and no radiographic evidence of occlusal caries, but with surface roughness, surface opacities, or staining. To our knowledge, its prevalence has not been quantified, which is the objective of this study. Methods: A total of 82 DPBRN dentist and hygienist practitioner-investigators from “The Dental Practice-Based Research Network” (DPBRN; www.DentalPBRN.org) participated, a network mainly comprising 5 regions: Alabama/Mississippi (AL/MS); Florida/Georgia (FL/GA); Minnesota (MN); Permanente Dental Associates (PDA); and Scandinavia (SK; Denmark, Norway, Sweden). When patients presented with at least one unrestored (no sealant or restoration) occlusal surface, the number of unrestored occlusal surfaces and the number of QOC were quantified. Consecutive patients 6 years or older were enrolled. Up to 2 lesions were recorded per patient. Information also was recorded about patient, tooth, lesion, diagnostic methods and treatments. Data were preliminarily analyzed using chi-square tests; additional hierarchical analyses will account for multiple lesions within patients and multiple patients within the same practitioner. Results: Overall, 7677 patients had at least one unrestored occlusal surface; the total number of unrestored occlusal surfaces was 50,445. Thirty percent (2301/7677) of patients had QOC; 9.5% (4,814/50,445) of surfaces had QOC. Patient- and tooth-level prevalence of QOC varied significantly by region (p<0.001): AL/MS had the highest patient-level prevalence at 39% (357/911); FL/GA 35% (641/1844); MN 34% (379/1113); SK 30% (755/2526); PDA had the lowest at 13% (169/1283). FL/GA had the highest tooth-level prevalence at 12.4% (1,378/11,105); AL/MS 10.0% (688/6,910); SK 9.5% (1,709/18,061); MN 9.3% (689/7,435); PDA had the lowest at 5.0% (350/6,934). Conclusions: To our knowledge, this is the first study to quantify the prevalence of QOC in routine clinical practice. These results document wide variations in prevalence among DPBRN's five main regions.