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Variations in leaf morpho-anatomy, chemical composition, gas exchange and water relations were studied in Corylus avellana L. cultivars 'Butler' and 'Segorbe', grown freely as multi-stemmed shrubs and as a free single trunk (free vase), during 2007. The trial was installed in 1980 with several commercial hazelnut cultivars in Northern Portugal, at 470 m above sea level. The leaf traits analysed included leaf mass per unit area (LMA), area, relative water content (RWC), succulence, the thickness of palisade and spongy parenchyma and epidermis, photosynthetic pigments, carbohydrates and total phenols concentrations. Net CO₂ assimilation rate, stomatal conductance, intercellular CO₂ concentration, transpiration rate, intrinsic water use efficiency and leaf water potential were also determined. Cultivar (cv.) 'Segorbe' grown on a single trunk and cv. 'Butler' grown freely presented the higher LMA. Trees grown on a single trunk had higher RWC than trees grown freely. Higher LMA presented by 'Butler' grown freely was associated with higher total lamina thickness. Higher total lamina thickness of 'Butler' on multi-stemmed was mainly due to a greater palisade parenchyma. The leaves of 'Butler' and 'Segorbe' grown freely, having a thicker palisade parenchyma, had higher values of gs and A. No significant differences in midday leaf water potential (Ψ_MD) were found between cultivars and training systems. Cv. 'Segorbe' on single trunk and cv. 'Butler' grown freely had leaves with higher total chlorophyll and total phenol concentrations. In addition, 'Segorbe' trees had higher carotenoid and starch concentrations. From a horticultural point of view it seems that cv. 'Butler' will benefit if grown freely and cv. 'Segorbe' on single trunk.