In this study, the effects of long-term nitrogen deficiency (N 0.1 mM for 4 months) on growth, phenolic content and activity of phenylalanine ammonia lyase (PAL; EC 126.96.36.199) were investigated in the leaves, inflorescences and roots of yarrow (Achillea collina Becker ex Rchb.) grown in hydroponics. The antioxidant capacity of methanol extracts was also evaluated. Nitrogen starvation decreased plant growth and the leaves’ total nitrogen, amino acids, proteins, chlorophylls and carotenoids contents indicating that the primary metabolism was severely limited by low nitrogen availability. The amount of total phenolics and the antioxidant capacity were higher in leaves and roots of nitrogen-starved compared to control plants. The most abundant phenolic acids identified in yarrow were 5-O-caffeoylquinic acid (chlorogenic acid), and 3,5 and 4,5-di-O-caffeoylquinic acids. Nitrogen starvation significantly increased the contents of all these compounds and the PAL activity in leaf and root tissues. Results suggest that hydroponics, with proper manipulation of nutritional factors, might be a suitable system for the production of compounds with health benefits, such as caffeic acid derivatives, in yarrow.