ABSTRACT Introduction: In patients with mechanical prosthetic heart valves or atrial fibrillation requiring anticoagulation to prevent thromboembolic events, several factors influence adherence and anticoagulation complications. Objective: To evaluate the factors that interfere with the quality and complications of anticoagulation with vitamin K antagonists. Methods: A retrospective cohort study of 100 patients, in the period from 2011 to 2014, was performed. Anticoagulation conditions in the last year, regarding the presence of complications (embolisms/bleeding) and inadequate treatment were assessed: achievement of less than 8 annual prothrombin times and International Normalized Ratio outside therapeutic target in more than 40% of prothrombin times. Results: There were 31 complications (22 minor bleeding without hospitalization and 9 major complications: 7 bleeding with hospitalization and two emboli); 70 were with International Normalized Ratio outside the target in more than 40% of the tests and 36 with insufficient number of prothrombin times. Socioeconomic factors, anticoagulant type and anticoagulation reason had no relationship with complications or with inadequate treatment. There were more complications in patients with longer duration of anticoagulation (P=0.001). Women had more International Normalized Ratio outside the target range (OR 2.61, CI:1.0-6.5; P=0.04). Patients with lower number of annual prothrombin times had longer times of anticoagulation (P=0.03), less annual consultations (P=0.02) and less dose adjustments (P=0.003). Patients with longer duration of anticoagulation have more complications (P=0.001). Conclusion: There was a high rate of major complications and International Normalized Ratio was outside the goal. Less annual prothrombin times was related to longer duration of anticoagulation, less annual consultations and less dose adjustments. More major complications occurred in patients with longer duration of anticoagulation.