The fluorescent dye indocyanine green (ICG) has been used clinically to evaluate cardiac output1 and hepatic function and in ophthalmic angiography for more than 3 decades. It shows peak spectral absorption at approximately 830 nm with an excitation wavelength of 800 nm. ICG binds tightly to plasma proteins and lipids and emits fluorescence signals in vascular structures, making it possible to observe blood flow conditions in both arterial and venous structures during vascular neurosurgery under a microscope. ICG has a half-life of 150e180 seconds and is removed from the circulation exclusively by the liver. This brief half-life permits repeat ICG injections and comparisons of blood flow status before and after neurosurgical procedures. The first clinical application of ICG, termed ICG video angiography (ICG-VAG), was described in 2003 by Raabe et al.2 ICG-VAG reveals gray-scale intensity changes in florescence signals over time and is used to confirm complete clipping of aneurysms, apart from patency of branch arteries and bypass grafts. Although currently ICG-VAG provides only qualitative or semiquantitative evaluation, we strongly believe that it is essential to establish quantitative evaluation by analyzing changes in ICG intensity over time. Previously published reports, including those from our laboratory, have suggested possible correlations between some of the parameters related to ICG-VAG analysis and actual blood flow measured with a transit-time flowmeter.3,4 Based on the foregoing factors, we believe that quantitative ICGVAG measurement will have a significant clinical impact in vascular brain surgery. Thus, we designed a 2-step study that consisted of basic experiments and clinical investigations of ICG-VAG to determine optimal parameters for quantitative measurement of blood flow.