The variability of heavy-duty diesel engine emissions was determined during a cooperative test program by the Engine Manufacturers Association and the Environmental Protection Agency. Six heavy-duty diesel engines with rated power ranging from 150 kW to 280 kW were evaluated to determine the nitrogen oxide, hydrocarbon and particulate emissions variability's within and among laboratories. Tests were performed using the recently adopted heavy duty diesel engine transient cycle and the 13-mode steady-state method which was used for certification until the 1984 model year. Transient and 13-mode test variability's were nearly the same for nitrogen oxides and particulates however hydrocarbon variability's were greater for transient than for 13-mode tests. The variances among different laboratories were greater than within individual laboratories by a factor of about two, except when several months of testing elapsed, in which case the variability within a laboratory was nearly the same as among-laboratories.Comparisons of variances and regulations indicated that nitrogen oxide and hydrocarbon variability's were about 25% of the regulated emission values. The corresponding particulate variability was 76% of the proposed regulation which suggests that particulate control presents a significant challenge.