Exhaust emissions from heavy-duty diesel engines operating at high altitude are of concern. EPA and Colorado Department of Health sponsored this project to characterize regulated and selected unregulated emissions from a naturally-aspirated Caterpillar 3208 and a turbocharged Cummins NTCC-350 diesel engine at both low altitude and simulated high attitude conditions (≈ 6000 ft). Emissions testing was performed over cold- and hot-start transient Heavy-Duty-Federal Test Procedure (HD-FTP) cycles as well as selected steady-state modes. In addition, the turbocharged engine was operated with mechanically variable and (fixed) retarded fuel injection timing to represent “normal” and “malfunction” conditions, respectively.High altitude operation generally reduced NOx emissions about 10 percent for both engines. Average composite HD-FTP emissions of HC, CO, particulate matter, and aldehydes measured at high altitude for the naturally-aspirated engine were 2 to 4 times the levels noted for low altitude conditions. The same emission constituents from the turbocharged engine at high altitude with “normal” timing were 1.2 to 2 times the low altitude levels, but were 2 to 4 times the low altitude levels with “malfunction” timing.