Blood flow restricted (BFR) exercise results in transient muscle growth that may be due to metabolite accumulation, hyperaemia and muscle damage, possibly serving as a stimulus for hypertrophy. Understanding the duration of this growth is important for exercise recovery and measurement of hypertrophy. To measure changes in muscle size via ultrasound throughout a 48-h period after a session of BFR knee extension (KE) exercise. Muscle thickness of the vastus lateralis (VL) was measured via ultrasound in 12 participants (six males and six females, age: 20.3±1.1years) before and immediately, 10min, 30min, 1, 3, 8, 24, and 48h after unilateral exercise. One leg served as a non-exercise control while the other leg performed four sets of unilateral BFR KE at 30% of one-repetition maximum with a pressurized cuff applied to the proximal thigh and inflated to 50% arterial occlusion pressure. Vastus lateralis thickness was 34.9±7.2% higher immediately after exercise, 28.6±7.9% at 10min, 25.2±6.1% at 30min, 14.9±4.8% at 1h and 11.8±5.6% at 3h (p<0.05). There were no changes compared to pre-exercise measurements past 3h, and the control limb did not change (p>0.05). The muscle thickness of the exercise leg was significantly greater than that of the control leg from immediately after exercise up to 1h post-exercise (p<0.05). Muscle thickness of the VL increases for 3h post-BFR exercise and returns to normal within 8h. This timeframe should be considered when prescribing exercise and planning muscle hypertrophy assessments.