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SUMMARY ‘Regrowth dieback’, a disease of Eucalyptus obliqua and E. regnans is causing serious degradation of high yield, even-aged, regrowth forest in south-eastern Tasmania. Dieback symptoms in crowns of diseased trees were first observed in the region in 1964 and since then the disease has continued to develop. Visual symptoms of the disease can be discerned only in crowns of dominant and co-dominant trees since sub-dominant and suppressed trees show similar symptoms due to normal stand competition. This work shows that for increment periods which finished in 1958 or earlier, basal area increments of dominant and co-dominant trees from a number of growth plots in the region were slightly lower in trees which were affected by the disease in 1971 than in those which were not. There were much larger reductions in increments of affected trees measured in increment periods which finished in 1960 or later. It was concluded that an agent very damaging to forest health appeared in the region in 1959 or early 1960. This coincides with the start of a period of several years of unusually dry summers in the region. These findings conform with an earlier suggestion that the disease may have been initiated by drought with secondary damage by the root-rotting fungus Armillaria sp. and/or the defoliating beetle Chrysophtharta bimaculata. The fungus is ubiquitous in the region and has frequently been associated with decayed roots of affected trees. The beetle is believed to have caused episodic tree defoliation in the region. It is suggested that the secondary agents normally cause death or ill-health of a few trees in otherwise healthy forest. Physiological weakening of trees by drought may have allowed the effects of these agents to increase substantially after 1959.