Recently, outbreaks of pine sawfly (Diprion pini [L.]) became one of the most relevant stresses resulting in widespread deterioration of Scots pine (Pinus sylvestris [L.]) conditions in Central and Northern Europe. At the beginning of the 1990s Lithuania experienced the largest outbreak of Diprion pini L. Field studies were carried out in middle-aged pine stands, exhibiting different degrees of damage caused by pine sawfly, in order to examine the resultant changes in Scots pine defoliation and death probability. “Mirror effect” in defoliation changes from tree decline to recovery was detected. If in the period of pest outbreak the largest pine trees, crown condition of which had been better than that of the other, suffered the most serious needle losses, then in the period of recovery these trees recovered most intensively. Intensity of tree mortality was in relation to stands mean defoliation degree. However, the relative diameter of dead trees did not differ among stands significantly (p>0.05) as suppressed trees died first. No threshold for crown defoliation, exceedance of which would inevitably result in Scots pine tree death, was found.