Previous solar observations have shown that coronal loops near 1 MK are difficult to reconcile with simple heating models .
These loops have lifetimes that are long relative to a radiative cooling time , suggesting quasi-steady heating .
The electron densities in these loops , however , are too high to be consistent with thermodynamic equilibrium .
Models proposed to explain these properties generally rely on the existence of smaller scale filaments within the loop that are in various stages of heating and cooling .
Such a framework implies that there should be a distribution of temperatures within a coronal loop .
In this paper we analyze new observations from the EUV Imaging Spectrometer ( EIS ) on Hinode .
EIS is capable of observing active regions over a wide range of temperatures ( Fe viii–Fe xvii ) at relatively high spatial resolution ( 1″ ) .
We find that most isolated coronal loops that are bright in Fe xii generally have very narrow temperature distributions ( \sigma _ { T } \lesssim 3 \times 10 ^ { 5 } K ) , but are not isothermal .
We also derive volumetric filling factors in these loops of approximately 10 % .
Both results lend support to the filament models .