Red blood cells (RBC) in normal human blood undergo reversible aggregation at low flow or stasis. The extent and kinetics of this phenomenon have been studied using various optical and electrical methods, yet results using such methods are not always in concordance. This study employed a horizontal glass tube in which blood flow could be established, then abruptly stopped. Normal blood and RBC suspensions with enhanced or decreased aggregation were studied. Light transmittance (LT) and electrical impedance at 100 kHz were recorded during high-shear flow and for 120 s after flow was abruptly stopped during which RBC aggregation occurs. Capacitance values were also obtained based on the imaginary part of impedance data and recorded. Various aggregation parameters were calculated, using the time course of LT, impedance, and capacitance, then compared with each other and with results from laboratory aggregometers. RBC aggregation parameters were calculated, using the time course of impedance data often failed to correlate with known changes of aggregation, even reporting aggregation for cells in nonaggregating media (i.e., RBC in buffered saline). Alternatively, RBC aggregation parameters based upon the time course of capacitance data are in general agreement with those derived from LT data and with RBC aggregation indexes, measured using commercial instruments.