©Operative Dentistry Objectives: This study was performed to 1) quantify dentists' treatment thresholds for occlusal primary caries; 2) determine if the patient's age affects dentists' decisions to surgically treat these carious lesions; and 3) test the hypothesis that patients', dentists', and practices' characteristics are significantly associated with surgical enamel intervention. Methods: The study used a cross-sectional design consisting of a questionnaire survey in Japan. This study queried dentists working in outpatient dental practices who were affiliated with the Dental Practice-Based Research Network Japan (JDPBRN), which aims to allow dentists to investigate research questions and share experiences and expertise (n=282). Participants were asked whether they would surgically intervene in a series of cases depicting occlusal caries. Each case included a photograph of an occlusal surface displaying typical characteristics of caries penetration and written descriptions of adult and pediatric patients at high caries risk. Results: In a case of a carious lesion within inner enamel, the proportion of dentists who indicated surgical intervention was significantly higher in adult patients (48%) when compared with pediatric patients (34%; p<0.01). Logistic regression analysis showed that using a dental explorer for the diagnosis of primary occlusal caries, type of practice, practice busyness, and percentage of patients who self-pay were significantly associated with dentists' decisions to intervene surgically into the inner enamel carious lesion. Conclusions: These findings demonstrate that more than one-third of participants chose to intervene surgically into inner enamel carious lesions, and patients' age affects dentists' decisions about when to intervene surgically (clinicaltrials.gov registration number NCT01680848).