The effect of green house on the phenolic content of olive leaves (Olea europaea L.) in seven genotypes was studied. A decrease of flavonoids was found in plants grown in green house in comparison to those grown in open air. This was found in all the genotypes. The mainly involved compounds are luteolin and luteolin-7-glucoside. When olive leaves are used as starting material for phytochemical extracts the ecological conditions under which plants are grown should attentively be considered.