The availability of good quality irrigation water is decreasing worldwide, and salinity is an increasingly important agricultural problem. To determine whether detrimental effects of NaCl can be minimized by additional Ca2+, tomato (Lycopersicon esculentum Mill.) `Super-sweet 100' was grown hydroponically. The basic nutrient solution contained 11.9 mM NO -3 and 3.2 mM Ca 2+. We added 14.1, 44.4 and 70.4 mM of NaCl to this solution to determine the effect of NaCl and there were treatments with 70 mM NaCl and 10 or 20 mM Ca2+ to look at Ca2+ effects. We also included three treatments in which all nutrient concentrations were increased (without NaCl) to distinguish between osmotic and ion-specific effects. 70.4 mM NaCl reduced leaf photosynthesis, chlorophyll content, gas phase conductance for CO2 diffusion, carboxylation efficiency, and dark-adapted quantum yield of photosystem II. The inclusion of 20 mM Ca2+ prevented these effects of NaCl. NaCl also decreased leaf size and elongation rate, but this could not be prevented by adding extra Ca2+ to the nutrient solution; these were caused by osmotic effects, rather than Na+ specifically. Likewise, plant dry mass was negatively correlated with solution EC, suggesting an osmotic effect. Our results indicate that leaf area development, which was inhibited by high EC, is more important for dry matter accumulation than leaf photosynthesis, which was inhibited by high Na+. Adding 20 mM Ca2+ to the 70 mM NaCl solution reduced the Na+ concentration in the leaf from 79 to 24 mg·g-1, which may explain why Ca2+ alleviates Na+ toxicity.