OBJECTIVES: Warfarin anticoagulation is monitored using international normalized ratio (INR); specified with the narrow/wide 2.0–3.0/1.9–3.5 therapeutic target range, and converted to time in the therapeutic range (TTR, total percentage of time the INR reading is within the INR therapeutic range). We explored the impact of I) INR monitoring and II) TTR on patients’ quality of life (QoL). METHODS: 647 unselected patients visiting 3 health centres in Pirkanmaa district (31.01–11.02.2011), Finland participated to prospective study. To assess INR monitoring or TTR and QoL, Beta regressions were applied in Stata and EQ-5D-3L-based QoL (Smithson&Verkuilen transformation) 3 months after the baseline visit was the dependent variable. The covariates were CHADS2 (congestive heart failure, hypertension, age>75, diabetes mellitus, stroke) score, other comorbidities, baseline QoL, time difference (days) between dependent variable measurement and mean TTR time point (subgroup model), and INR monitoring (yes/no in total population) or TTR % (subgroup model). RESULTS: 28 patients (46.43% male; means: age 73.21 years, CHADS2-score 2.39, other comorbidities amount 1.71, baseline QoL 0.8334, INR tests 4.21, 66.31%/82.86% INR measurements on the narrow/wide range) had INR measurements ("warfarinization group") during the 3 months follow-up. 27 patients had calculable TTR (Rosendaal method) that was 69.29%/86.66% on the narrow/wide range. In the beta regression (N 393, +1 marginal change), CHADS2-score (-0.021), other comorbidities amount (-0.030), baseline QoL (+0.350), and INR monitoring (p=0.299, -0.042, SE 0.044) predicted QoL. The mean adjusted QoL with/without INR monitoring in the warfarinization-like group was 0.794/0.836. In subgroup Beta regression including only warfarinization patients, higher TTR levels predicted lower QoL (p<0.050, +1 %-unit marginal change -0.006 [SE 0.007]; discrete change -0.284 in the wide range from 35.3% to 100.0%). CONCLUSIONS: INR monitoring may predict QoL loss and, surprisingly, higher TTR predicts lower QoL. Larger studies are needed to confirm the potential relationship between TTR and QoL.