  We consider the orbital ordering in LaMnO3 and similar systems, proceeding
from the band picture. We show that for the realistic magnetic structure of
A-type there exists a complete nesting betweeen two e_g-bands. As a result
there occurs an instability towards an excitonic insulator-like state -- an
electron-hole pairing with the wave vector Q=(\pi,\pi), which opens a gap in
the spectrum and makes the system insulating. In the resulting state there
appeasr an orbital ordering -- orbital density wave (ODW), the type of which
coincides with those existing in LaMnO3.
