Photosynthesis, stomatal conductance, electron transport, internal CO2 and sugar levels were determined in the leaves of Yellow Canopy Syndrome (YCS) symptomatic sugarcane (Saccharum spp.) plants. Two varieties from two different geographic regions in Australia, KQ228 and Q200 were used. Although visual yellowing was only evident in the lower leaves of the canopy (older than Leaf 5), photosynthesis and stomatal conductance were lower in both the yellowing leaves and those not yet showing any visible symptoms. On a canopy basis, photosynthesis was reduced by 14% and 36% in YCS symptomatic KQ228 and Q200 plants, respectively. Sucrose levels increased significantly in the leaves, reflecting some of the earliest changes induced in YCS symptomatic plants. The electron transport characteristics of dark-adapted leaves showed disruptions on both the electron acceptor and donor side of PSII. Some of these changes are characteristic of a degree of disruption to the protein structure associated with the electron transport chain. Based on the results, we propose that the first change in metabolism in the YCS symptomatic plants was an increase in sucrose in the leaves and that all the other changes are secondary effects modulated by the increased sugar levels.