In many countries there is scarce information on the impact of prescribed burns and forest fires, despite its importance for fire management, hence the objective of the present investigation, conducted on the Ajusco volcano at Mkico, D.F., was to study the effect of fire on the mortality and relative increment in height of young Pinus hartwegii Lindl., two years after the application of the treatments. The treatments consisted of prescribed burns carried out at two periods of 2002 (March 21 and May 29), at two intensities (high and low), in two conditions of stands with young trees of Pinus hartwegii (300-700 trees ha-1, with grasses as most abundant fuel; 900-2500 trees ha-1, with needle litter as most abundant fuel). An analysis was made of the effect of the considered treatments on mortality, through logistic regression, and its effect was determined on relative height increment with the t test. The May burns caused higher mortality than those of March (p£0.0001); as did those of high intensity with respect to the low intensity burns (p£0.0001); and the areas of high density, with respect to those of low density (p£0.0001). The smaller the tree diameter to the breast height, the higher was the mortality (p£0.0001). According to the resulting model, a tree with a normal diameter of 5 cm, growing at high density, which is affected by high intensity fire in May, has a probability of mortality of 0.90, whereas for a tree of equal diameter growing at low density and affected by low intensity fire in March, the probability of mortality will be 0.01. The experiment was carried out over a two year period. The relative height increment after the application of the fires was greater in the areas of low density, burned in March at low intensity (11.4 cm m-1 the first year and 8.1 cm m-1 the second year) compared to the control at low density (6.5 cm m-1 the first year and 4.8 cm m-1 the second year) (p=0.0067 and 0.0001, respectively).