We analyze the stability of relativistic, quasi-equilibrium binary neu
tron stars in synchronized circular orbit. We explore stability agains
t radial collapse to black holes prior to merger, and against orbital
plunge. We apply theorems based on turning points along uniformly rota
ting sequences of constant angular momentum and rest mass to locate th
e onset of secular instabilities. We find that inspiraling binary neut
ron stars are stable against radial collapse to black holes all the wa
y down to the innermost stable circular orbit.