To maximise the potential of the tree-ring isotopic signal for palaeoclimate research it is essential to understand and characterise the natural variability between individual trees. This study explores the nature of inter-tree isotopic variability and evaluates the implications for developing robust palaeoclimate reconstructions. We confirm levels of natural inter-tree variability similar to those reported in previous studies, but demonstrate, using a large data set of isotopic measurements determined from individual rings of 100 trees, that to obtain a representative regional environmental signal and to reduce problems when combining records, higher levels of replication than those typically adopted in isotope dendroclimatology may need to be considered.