Biodiesel has been used to reduce petroleum consumption and pollutant emissions. B20, a 20% blend of biodiesel with 80% petroleum diesel, has become the most common blend used in the United States. Little quantitative information is available on the impact of biodiesel on engine operating costs and durability. In this study, eight engines and fuel systems were removed from trucks that had operated on B20 or diesel, including four 1993 Ford cargo vans and four 1996 Mack tractors (two of each running on B20 and two on diesel). The engines and fuel system components were disassembled, inspected, and evaluated to compare wear characteristics after 4 years of operation and more than 600,000 miles accumulated on B20. The vehicle case history-including mileage accumulation, fuel use, and maintenance costs-was also documented. The results indicate that there was little difference that could be attributed to fuel in operational and maintenance costs between the B20-and diesel-fueled groups. No differences in wear or other issues were noted during the engine teardown. The Mack tractors operated on B20 exhibited higher frequency of fuel filter and injector nozzle replacement. Biological contaminants may have caused the filter plugging. A sludge buildup was noted around the rocker assemblies in the Mack B20 engines. The sludge contained high levels of sodium, possibly caused by accumulation of soaps in the engine oil from out-of-specification biodiesel. The Mack and Ford engines used similar pump-line nozzle fuel injection systems, but a much larger volume of fuel was recirculated in the larger Mack engines. This, along with duty cycle and engine loading, may account for the difference in performance of the two engine types operated on B20.