Over the decades after the introduction of microvascular surgery, free-flap transfer has been one of the options for the reconstruction of soft-tissue defects caused by trauma, cancer resection, and such. The main adverse event of free-flap transfers is vascular complication, leading sometimes to flap loss. With advances in microvascular surgery, methods of flap monitoring have also been developed: surface temperature scanning,[ 1 ] implantable or transcutaneous Doppler,[ 2 ] [ 3 ] [ 4 ] blood-flow monitoring with intravenous catheter,[ 5 ] transcutaneous laser Doppler,[ 6 ] [ 7 ] [ 8 ] [ 9 ] microdialysis,[ 10 ] [ 11 ] transcutaneous pO₂ monitoring,[ 12 ] optical spectroscopy,[ 13 ] [ 14 ] endscopy,[ 15 ] and such. The sensitivity and specificity of these methods are different; however, high sensitivity should be given priority over high specificity for good clinical outcomes. Furthermore, monitoring methods need to be simple, easy to perform, and cost-effective. Here, we propose the simplest and most cost-effective method of flap monitoring.