According to the hypotheses advanced by Wardle (1971) and Tranquillini (1979), failure of trees at their altitudinal limit is caused by poor development of the cuticle and a corresponding inability to conserve water in winter when the soil is frozen. This hypothesis was tested at a natural tree line of Scots pine in the Cairngorm mountains of Scotland. There was no sign of poor cuticular development, but needles from oldkrummholz at high altitudes and young isolated trees at higher altitude did lose water more rapidly than those at low altitude, over a range of water content during desiccation in the laboratory. The results are best attributed to stomatal dysfunction caused by mechanical damage to the leaf and also by direct damage to the cuticle, rather than to a thinner or less developed cuticle. Calculations show that a small increase in cuticular transpiration is quite unlikely to cause frost-drought.