The Keetch - Byram Drought Index ( KBDI ) is used for assessing drought for the needs of planning fire management operations. Drought has a well documented effect on plant flammability. Plants are water stressed in periods of drought. Plant water potential is a go od measure of plant water stress. Measurement of water potential is possible using a specialized device, the pressure chamber instrument. KBDI calculation requires a long series of meteorological data . When the data are available its calculation is straigh tforward . H owever , it is strictly valid for the location of the meteorological station and its immediate vicinity. On the other hand, plant water potential can be measured at any place and does not require long - term monitoring of a site of interest, but it s measurement needs a specialized instrument and is cumbersome. Establishing a relationship between KBDI and plant water stress can be useful for fire management. The existence of such a relationship was investigated in this study. The investigation was based on systematic measurements of the water potential ( O ) of three representative plant species ( Pinus halepensis , Quercus coccifera , and Cistus creticus ), on the south slopes of Hymettus mountain near Athens, through the fire seasons of 2003, 2004 and 2 005. The instrument used was a Scholander pressure chamber. Water potential values were then associated with KBDI values for the area. KBDI was calculated with data from the nearest meteorological station that is located at a distance of 6.3 km from the sa mpling site. The analysis resulted in the development of three linear regression equations with water potential as the dependent variable and KBDI as the independent. The equations are all significant (P - values < 0.001) although their adjusted R 2 values ar e relatively low, being equal to 0.285, 0.486, and 0.411 respectively for the three species. Further analysis, adding the 5 - day average of mid - day relative humidity (RH) (%) as an independent variable, resulted in improved equations for Quercus coccifera a nd Cistus creticus (adjusted R 2 equal to 0.610 and 0.550 respectively). In conclusion it appears that the KBDI is not only an index characterizing cumulative moisture deficiency in deep duff and upper soil layers, and consequently the flammability of dead organic matter, but also reflects to some extent water deficit in living plants, and hence their flammability. This finding may provide a new insight into the relation of high KBDI values with intense fire behaviour .