Questionable occlusal carious lesions (QOC) can be defined as an occlusal tooth surface with no cavitation and no radiographic radiolucencies, but caries is suspected due to roughness, surface opacities or staining. An earlier analysis of data from this study indicates 1/3 of patients have a QOC. The objective of this report has been to quantify the characteristics of these common lesions, the diagnostic aids used and the treatment of QOC. A total of 82 dentist and hygienist practitioner-investigators from the USA and Denmark in the National Dental Practice-Based Research Network participated. When consenting patients presented with a QOC, information was recorded about the patient, tooth, lesion and treatments. A total of 2,603 QOC from 1,732 patients were analyzed. The lesions were usually associated with a fissure, on molars, and varied from yellow to black in color. Half presented with a chalky luster and had a rough surface when examined with an explorer. There was an association between color and luster: 10% were chalky-light, 47% were shiny-dark and 42% were mixtures. A higher proportion of chalky than of shiny lesions were light (22 vs. 9%; p < 0.001). Lesions light in color were less common in adults than in pediatric patients (9 vs. 32%; p < 0.001). Lesions that were chalky and light were more common among pediatric than among adult patients (22 vs. 6%; p < 0.001). This is the first study to investigate characteristics of QOC in routine clinical practice. Clinicians commonly face this diagnostic uncertainty. Determining the characteristics of these lesions is relevant when making diagnostic and treatment decisions.