The study characterized the influence of cucumber mosaic virus (CMV) on the growth response of two annual weeds to nitrogen. Plants were grown individually along a N gradient from 4 to 32 mmol l−1 and data were interpreted using growth analysis. Plant biomass increased with N concentration and was significantly higher for healthy than infected plants at the two highest N concentrations. Healthy plants of Portulaca oleracea L. were characterized by lower biomass allocation to leaves and higher biomass allocation to roots than infected plants; no change in biomass allocation was recorded for Stellaria media Vill. Relative growth rate (rgr), net assimilation rate (nar) and specific leaf area (sla) of plants increased with increasing N concentration. Healthy plants of P. oleracea were characterized by a higher rgr and nar and a lower sla than infected plants, whereas healthy S. media had a higher rgr but a similar nar and sla or leaf weight ratio (lwr) compared with infected plants. The consequences of these results on the population dynamics of weeds and virus spread are discussed.