Young vegetative Brussels sprout and leek plants were grown in a growth chamber at a light intensity of 550 micro mol m-2 s-1 or at only 20% of that intensity. In both light treatments, Brussels sprouts had a relative growth rate (RGR) ~90% higher than that of leeks, which was mainly explained by a higher leaf area ratio (LAR; msuperscript 2/kg plant). Only minor differences in the physiological component (net assimilation rate) were observed between the 2 species within a light treatment. The higher LAR of Brussels sprouts was mainly explained by higher specific leaf area. Brussels sprouts had a higher rate of biomass production per unit internal N (N productivity) than leeks. This was mainly explained by a higher allocation of N to leaves and a higher rate of biomass production per unit leaf N. It is suggested that the higher biomass production per unit of N taken up in Brussels sprouts than leeks as observed in the field is explained by higher N productivity. The results obtained from the growth chamber studies are discussed in relation to field experiments and data from the literature.