Since a large area of Italy is lacking of updated informations about weed flora, we carried out a field survey in wheat and corn crops in western-central Italy (Lazio), by phytosociological approach, to update data, making comparisons and diachronic observations  with literature. Relevés were carried out in 62 wheat crops, 50 corn ones. Totally 149 vascular units and 36 families were recorded; Poaceae and Asteraceae are prevailing. In wheat fields 113 units were observed: Dicots s.l. are 77,88% and Monocots 22,12%; in corn fields 78 units were observed. Floristic composition of relevés is very variable. In both cultivations Therophytes, Mediterranean and wide distribution taxa dominate. Among wheat weeds one alien species was observed; greater (11%) was the occurrence of non-native taxa in corn crops. The comparison of the surveys in 1981 and in 2011-2012 showed the prevailing of Asteraceae, Fabaceae, Brassicaceae, Poaceae, even if in different percentages. A strong increase of Therophytes and Mediterraneaneans is detected. Wheat crops weed flora has a higher value than that in corn crops, because of species richness and mostly native taxa presence. The prevailing of Dicots, not resistent to herbicides, can be considered an indicator of good conservation status of agro-ecosystems.