The aim of this study is to investigate the parameters influencing the real driving emission monitoring with particular attention towards the influence of road gradient. For this purpose, an experimental activity was carried out with a Euro 5 Diesel light-duty vehicle, driven along two tracks of Naples characterized by a different road gradient: the first pattern is quite flat, the second includes positive (+2.9%) and negative (−3.6%) road gradient.Exhaust emissions of CO, THC, NOx, CO2 were acquired on road by using a portable emission measuring system (PEMS) connected also to the Engine Control Unit for saving the main engine parameters and to the GPS for the geographical coordinates and altitude. The acquired speed profiles were repeated on the chassis-dynamometer without simulating the road gradient. In such way, comparison between on road and laboratory results over the flat pattern allowed to identify the main differences among the two testing procedures for pollutant emission measurements. Moreover, the influence of slope variability was statistically evaluated by comparing laboratory and on-board results of the pattern in altitude.Results highlight a good correlation of emission monitoring during on-road and laboratory tests for all the analyzed pollutants with the exception of NOx which are strongly influenced by the operating engine temperatures. Slight differences of intake air temperature (almost 10 degC) can provide great difference of NOx exhaust emissions. A negative road gradient could cause a reduction higher in magnitude than the increase caused by a positive road gradient compared to a gradient-neutral test.