ENVI does support Erdas IMG format and no header file should be required. Are you using ENVI (new interface) or ENVI Classic? What is the error message you receive?

Thanks, that's what I thought which is why I'm not sure! I'm using Envi IDL+ Classic 5.2 (trying to open with classic. this is the only version of ENVI that works for me since I updated my mac to Yosemite 10.10) and the error message is: File: /pathname/lidarfile.img Error reading ERDAS file: Compressed ERDAS files are currently not supported. The file was compressed into a .7z file but I unzipped it before opening.

Also I can open other hyperspectral imagery files with headers with this version of ENVI

ERDAS .img files often come with an internal compression. For example, saving a file from ArcGIS to IMG format usually has a compression. Unfortunately, ENVI Classic does not support this compression but ENVI (new interface) does. So I would suggest opening the .img in ENVI (new interface of 5.x) and then save it to ENVI format for ease of use. ENVI 5.2 should work on Yosemite but you may need to apply a couple of 'tweaks': http://www.exelisvis.com/Support/Help...

Unfortunately I don't think these files were inherently collected to be used in erdas, but to be honest I'm not sure how they were initially compressed as I'm using them from a public repository years later. When you say open in the new interface 5.x, which interface are you referring to? I have 5.1 and 5.2 and it's not working on either. I can try opening them in another interface altogether and converting to geotif, but would've preferred to stay in the same software package for simplicity. Also another unfortunate thing is that I've tried all of those tweaks and still no dice :( I'm trying to log-in through a server that hosts ENVI and it never launches. I may be in the small population of people it's just not working for. I'll keep trying though just on the offhand chance I've installed something wrong/outdated but doesn't look that way :/ Thanks for all the help thus far!

You said you were using ENVI Classic, which does not support compressed ERDAS files. ENVI (the new interface) that you use starting with 5.0 does support compressed ERDAS .img files. So you really need to get this version working. Are you trying to log in to a remote Mac system using SSH? You might try (from X11) to CD into the /Applications/exelis/envi52/bin directory and sourcing the envi_setup.bash: >source envi_setup.bash then try to launch ENVI: >envi