| L | Source code (Lisp) |
| LAB | Label file (Visual dBase) |
| LAN | Loadable module (LAN DLL) (NetWare) |
| LAW | Electronic form file from eLaw Publishing. |
| LAY | Clipart file (Printmaster Gold) |
| LBG | Label generator data (dBase IV) |
| LBL | Label (Clipper 5) |
| LBM | Linear Bitmap graphics (XLib) |
| LBO | Compiled label (dBase IV) |
| LBR | Display driver (Lotus 1-2-3) |
| LBT | Labels (Microsoft FoxPro) |
| LBX | Labels (Microsoft FoxPro) |
| LCF | Linker control file (Norton Guides compiler) |
| LCH | Used in a program (unknown) that monitors a network's response time |
| LCK | Lockfile (Paradox) |
| LCL | Datafile (FTP) |
| LCN | Lection document (WordPerfect for Windows) |
| LCS | Data History file (ACT!) |
| LCT | Loader Control File In Oracle Applications Environment |
| LCW | Spreadsheet (Lucid 3-D) |
| LD | Long distance area codes file (Telix) |
| LD1 | Overlay file (dBase) |
| LDB | Lock file (Microsoft Access) |
| LDF | Library definition file (Geoworks Glue) |
| LDF | Microsoft SQL Server log file |
| LDL | Library (Corel Paradox) |
| LDS | LOUDNESS Sound System File |
| LDT | Loader Data File in Oracle Applications Environment |
| LEG | Legacy document |
| LEMON | Renamed RAR compressed archive file |
| LES | System game profiles (same as REG file) (Logitec Entertainment) |
| LET | Genesis 2000 mortgage application file |
| LET | Letter (generic) |
| LET | Unknown photo image |
| LET | Broderbund CreataCard Letterhead file |
| LEV | Level file (NetHack 3.x) |
| LEX | Adobe Linguistic Library data file |
| LFD | Data resource file (LucasArts Dark Forces) |
| LFP | LaserForms Plus file (Evergreen) |
| LFT | Laser printer font (ChiWriter) |
| LG | Logo procedure definition (LSRHS Logo) |
| LGC | Application log file |
| LGD | Application log file |
| LGO | Header and footer logo (SuperFax) |
| LHA | Compressed Archive (LHA/LHARC) |
| LHW | Compressed Amiga archive (LHWARP) |
| LIB | Library file (Generic) |
| LIF | Compressed archive (Generic) |
| LIM | Compressed archive (LIMIT) |
| LIN | Interactive music sequencing data file (Electronic Arts) |
| LIS | Listing (VAX) |
| LIT | eBook file for Microsoft Reader |
| LIX | Logos library system file |
| LJ | Text file (Hewlett-Packard LaseJet II printer) |
| LK | Database file (OpenSight-16 bit) |
| LKO | Linked object (Microsoft Outlook Express Junkmail file) |
| LL3 | Document file (LapLink III) |
| LLX | Exchange agent (Laplink) |
| LNK | Linker response file (RTLink) |
| LNK | Shortcut file (Microsoft Windows 9.x) |
| LNT | LiveNote Transcript file |
| LOD | Load file (Generic) |
| LOG | Log file (Generic) |
| LOK | Compressed file (FileWrangler) |
| LP | A document reader used for downloading mortgage closing information (DesertDocs) |
| LPC | Printer driver (TEKO) |
| LPD | Helix Nuts and Bolts file |
| LPI | Information file for laser printers (common with some scanning sofware) |
| LRC | Video phone file (Intel) |
| LRF | Linker response file (Microsoft C/C++) |
| LRF | Line Rider custom font file |
| LRS | Language resource file (WordPerfect for Windows) |
| LSF | Logos library system file |
| LSL | Script library (Lotus) |
| LSP | AutoLISP, CommonLISP, and other LISP language files |
| LSS | Spreadsheet (Legato) |
| LST | Keyboard macro (1st Reader) |
| LTM | Form (Lotus Forms) |
| LU | Library unit file (ThoughtWing) |
| LVL | Miner Descent/D2 Level extension (Parallax Software) |
| LWA | Lightwave Scene (version 6 and up) |
| LWD | Text document (LotusWorks) |
| LWF | Wavelet graphics file (Luratech) |
| LWLO | Layered Object file (Lightwave) |
| LWO | Lightwave Object (version 6 and up) |
| LWOB | Object file (Lightwave) |
| LWP | Wordpro 96/97 file (Lotus) |
| LWSC | Scene file (Lightwave) |
| LWZ | Linguistically enhanced sound file (Microsoft) |
| LYR | Layer file (DataCAD) |
| LZD | Difference file for binaries (Ldiff 1.20) |
| LZH | Compressed archive (LH ARC) |
| LZS | Data file (Skyroads) |
| LZW | Compressed Amiga archive (LHWARP) |
| LZX | Compressed archive (Generic) |
