  The existence of low-mass sterile neutrinos is suggested by the current
status of solar and atmospheric neutrinos together with the LSND experiment. In
typical four-flavor scenarios, neutrinos would contribute to a cosmic hot dark
matter component and to an increased radiation content at the epoch of
matter-radiation equality. These effects leave their imprint in sky maps of the
cosmic microwave background radiation (CMBR) and may thus be detectable with
the precision measurements of the upcoming MAP and PLANCK missions.
