Achieving therapeutic anticoagulation with warfarin is complicated by substantial inter-patient and intra-patient variability with numerous factors known to influence dose requirements. Obesity is one factor for which there remains no study to date investigating its initial effect on warfarin response assessed by INR, stratified by BMI category in hospitalized patients. To compare initial warfarin response between obese and non-obese patients by evaluating average daily dose (ADD), time required to attain therapeutic INR, and mean discharge dose (MDD), stratified by BMI category. A retrospective review was conducted to evaluate initial warfarin response in hospitalized patients of different BMI categories initiated on warfarin with ≥4 consecutive days of therapy and managed by pharmacy dosing service. 211 patients were included (10 underweight, 45 normal weight, 48 overweight, 71 obese, 37 morbidly obese). Across BMI categories, the percentage of patients attaining therapeutic INR prior to discharge differed (p = 0.0004) with 71.1 % of normal weight therapeutic compared to 42.3 % of obese and 38 % of morbidly obese. Within BMI categories, when comparing ADD between patients therapeutic and subtherapeutic at discharge, no differences were observed, except among overweight patients (5.6 ± 0.3 vs. 7 ± 0.4 mg, p = 0.0143). Compared to normal weight, obese and morbidly obese required a significantly longer median time to achieve therapeutic INR (8 and 10 days vs. 6 days) and a higher ADD (6.6 ± 0.3 and 7.6 ± 0.5 vs. 5 ± 0.3 mg) and MDD (6.7 ± 0.5 and 6.7 ± 0.7 vs. 4.4 ± 0.5 mg). Compared to normal weight, obese and morbidly obese patients had a decreased initial response to warfarin.