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Background: Although activated partial prothrombin time (aPTT) has often been used as a biomarker for evaluating the safety of dabigatran use in patients with non-valvular atrial fibrillation (NVAF), the optimal frequency of aPTT measurements is unclear. This study aimed to identify the frequency distribution of aPTT measurements in clinical practice and its clinical significance. Methods: This was a retrospective cooperative study conducted in 2 sites. All NVAF patients who underwent aPTT measurements before and after dabigatran treatment were included (n=380). The patients were divided into 2 groups according to the frequency of aPTT measurements during the first 3 months after drug prescription: Group A: infrequent group with only 1 measurement; and Group B: frequent group with ≥2 measurements. The clinical characteristics and outcomes were compared between the groups. Results: The frequency of aPTT measurements in the 3 months after dabigatran initiation varied: 240 patients underwent 1 measurement (Group A), and the remaining 140 patients underwent repeated measurements (Group B). There were significant differences in age and creatinine clearance (Ccr) between the groups (Group A vs. Group B: age 64.0±11.7 vs. 67.0±11.1 years, p=0.01; Ccr 83.8±30.3 vs.76.7±31.1 mL/min, p=0.03). During the mean follow-up period of 310 days, there were no significant differences in the discontinuation rate and incidence of bleeding (17% vs. 15% and 5% vs. 3%, respectively; both not significant). In Group B, the aPTT rarely increased beyond twice the upper normal limit within the 3 months (2.1%), although the correlation between the initial and subsequent aPTT measurements was low (r=0.366). Conclusions: In this retrospective study, the frequency of aPTT measurements after dabigatran initiation might have been dependent on patient characteristics. However, frequent aPTT measurements did not lead to a reduction in adverse clinical events.