The main characteristics of current annual diameter increment of the analysed narrow-leaved ash (Fraxinus angustifolia Vahl) future trees are variability and oscillations that can be ascribed to a different tree structure and the effect of exogenous factors. The elements that determine the increment potential are readily measurable and visible parameters of tree structure. The significant favourable exogenous factors affecting increment are average water level throughout the year, air temperature during the growth season, while rainfall during the period May - June is unfavourable. Marked depressions of diameter increment can be ascribed to the impact of defoliators.