We reviewed the effects of elevated ozone (O3), alone and in combination with elevated carbon dioxide (CO2) on primary and secondary metabolites of trees and performance of insect herbivores by means of meta-analysis. Our database consisted of 63 studies conducted on 22 species of trees and published between 1990 and 2005. Ozone alone had no overall effect on concentrations of carbohydrates or nutrients, whereas in combination with CO2, elevated O3 reduced nutrient concentrations and increased carbohydrate concentrations. In contrast to primary metabolites, concentrations of phenolics and terpenes were significantly increased by 16% and 8%, respectively, in response to elevated O3. Effects of ozone in combination with elevated CO2 were weaker than those of ozone alone on phenolics, but stronger than those of ozone alone on terpenes. The magnitude of secondary metabolite responses depended on the type of ozone exposure facility and increased in the following order: indoor growth chamber <open-top chamber <free-air ozone exposure. The results of meta-analysis demonstrated that, in terms of leaf chemistry, angiosperms are more responsive to elevated O3 than gymnosperms, as shifts in concentrations of carbohydrate and phenolics were observed in the former, but not in the latter. Elevated O3 had positive effects on some indices of insect performance: pupal mass increased and larval development time shortened, but these effects were counteracted by elevated CO2. Therefore, despite the observed increase in secondary metabolites, elevated O3 tends to increase tree foliage quality for herbivores, but elevated CO2 may alleviate these effects. Our meta-analysis clearly demonstrated that effects of elevated O3 alone on leaf chemistry and some indices of insect performance differed from those of O3+CO2, and therefore, it is important to study effects of several factors of global climate change simultaneously.