The recent financial crisis has highlighted the fragility of the US (and other countries') financial system under several respects. In this paper, the properties of a summary index of financial fragility, obtained by combining information conveyed by the "Agency," "Ted" and "BAA-AAA" spreads, timely capturing changes in credit and liquidity risk, distress in the mortgage market, and corporate default risk, are investigated over the 1986-2010 period. The empirical results show that observed fluctuations in the financial fragility index can be attributed to identified (global and domestic) macroeconomic (20%) and financial disturbances (40% to 50%), over both short- and long-term horizons, as well as to oil-supply shocks in the long-term (25%). The investigation of specific episodes of financial distress, occurred in 1987, 1998 and 2000, and, more recently, over the 2007-2009 period, shows that sizable fluctuations in the index are largely determined by financial shocks, while macroeconomic disturbances have generally had a stabilizing effect.