We describe a search for diffuse emission from several candidate sources of gamma-rays with energies ≳ 100 TeV, using the Chicago Air Shower Array / Michigan Muon Array (CASA-MIA) experiment. Since March 1990, CASA-MIA has recorded over 2.0 × 109 air shower events. Searches for diffuse emission are based upon the muon content of air showers. Candidate source regions include molecular clouds in the galaxy and the galactic plane. Results from CASA-MIA are compared to predicted diffuse emission.