Scots pine populations in the SE "Sistema Ib,rico" range suffered a severe defoliation in the S face of the crown in isolated trees of thinned stands. This process was detected at the end of the winter 2001-2002. We hypothesise that winter conditions may be as critical for Scots pine survival as summer drought, even for these southern populations. In this way, the possible connection between the microclimate conditions and the risk of xylem embolism during the winter of 2001-2002 was analysed. The additional decrease in water potential found in the affected trees could be caused by the combination of (1) low soil temperatures limiting water uptake by roots, (2) a higher vulnerability to drought-induced embolism due to the occurrence of repeated freeze-thaw cycles, and (3) high radiation events throughout the winter, increasing the loss of water by transpiration. Thus, the additional induction of embolism found in the affected trees (ca. 27%) could be caused by the combination of these factors. Estimated conditions during winter 2001-2002 were extremely unfavourable, leading probably to an impaired water status and high embolism rates, which may have induced the severe defoliation observed in crowns of affected trees.