OBJECTIVES: To investigate the underprescription of oral anticoagulation (OAC) in individual atrial fibrillation (AF) patients in primary care. SETTING: Screening of patient records in 39 participating general practitioners (GPs) across the Netherlands. PARTICIPANTS: We screened 101 207 patient records identifying 2375 non-valvular AF patients. METHODS: Using electronic patient files, we were able to screen the entire GP population for AF, CHA2DS2-VASc stroke risk scores, and the use of guidelines recommended OAC prescription. In case of a deviation from guidelines recommended OAC prescription, we checked the electronic patient file for any documented reason. Additionally, 6 weeks following the screening, we asked all GPs to provide information on any actions taken for the underprescribed patients. RESULTS: We found a mean CHA2DS2-VASc score of 3.2. OAC prescription consisted of direct OAC in 1342/1984 (68%) and vitamin K-antagonists in the remainder of patients. OAC underprescription was present in 93/944 (9.9%) females and 101/1374 (9.7%) in males, respectively. In 111/146 (76.0%) of the underprescribed AF patients, no reason to withhold OAC was reported. Reported reasons to withhold OAC were patient refusal (n=10), cardiologist advice (n=7) and high risk of bleeding (n=7). Data regarding actions following the identification of OAC underprescription were available for 92/194 (47%) of the OAC underprescribed cases. After consultation OAC was initiated in 9/92 (10%) only. CONCLUSIONS: In this large Dutch study among GPs, we observed 9.8% underprescription of OAC in AF patients. In 76% of the AF patients lacking a prescription for OAC, no documentation for deviating from the guidelines was found. Only in a minority of cases detection of OAC underprescription lead to OAC initiation.