Leaf morphology varies reliably with increasing altitude in many species, and this is generally considered to be related to temperature. Changes in irradiance with elevation may confound any relationships between a morphological character and altitude, particularly if altitude of origin affects the response to irradiance. Here we describe the interaction between irradiance and altitude of origin on leaf morphology of Southern beech, Nothofagus cunninghamii. Cuttings from each of four altitudes were grown in a glasshouse under full sunlight or 50% shade, and leaf morphology was related to irradiance, altitude of origin and accession. There was a significant interaction between irradiance and altitude of origin for leaf length, width, thickness, area, weight, specific leaf area and stomatal density. There was no effect of altitude on leaf length to width ratio or stomatal index, nor was there an interaction between irradiance and altitude of origin for these variables. These results show that the altitude of origin of a plant has an overriding impact on the leaf morphological response to irradiance. This must be considered in climatic reconstructions.