BACKGROUND: 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. PURPOSE: To measure changes in muscle size via ultrasound throughout a 48-h period after a session of BFR knee extension (KE) exercise. METHODS: Muscle thickness of the vastus lateralis (VL) was measured via ultrasound in 12 participants (six males and six females, age: 20.3 ± 1.1 years) before and immediately, 10 min, 30 min, 1, 3, 8, 24, and 48 h 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. RESULTS: Vastus lateralis thickness was 34.9 ± 7.2% higher immediately after exercise, 28.6 ± 7.9% at 10 min, 25.2 ± 6.1% at 30 min, 14.9 ± 4.8% at 1 h and 11.8 ± 5.6% at 3 h (p < 0.05). There were no changes compared to pre-exercise measurements past 3 h, 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 1 h post-exercise (p < 0.05). CONCLUSION: Muscle thickness of the VL increases for 3 h post-BFR exercise and returns to normal within 8 h. This timeframe should be considered when prescribing exercise and planning muscle hypertrophy assessments.