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Four 10- to 20-year-old plantations were precommercially thinned to determine the effects on tree growth and mortality caused by armillaria and heterobasidion root diseases. The plantations represented different species compositions with one each of (1) coastal Douglas-fir and noble fir, (2) Douglas-fir and western hemlock, (3) pure Douglas-fir, and (4) Shasta red fir and mountain hemlock. After 30 years, the probabilities of leave-tree survival and actual leave-tree survival (trees/ha [TPH]) were significantly (P ≤ 0.05) higher in thinned versus unthinned plots in one of the four plantations with no significant differences in the other three plantations. Most tree mortality was caused by armillaria root disease. Despite the high frequency of Heterobasidion occidentale in overstory stumps, only two leave trees in one plantation were killed by this fungus after 30 years. Quadratic mean diameter (QMD) growth and basal area (BA) (per ha) growth of leave trees were significantly (P ≤ 0.05) greater in thinned than in unthinned plots in one plantation for QMD and in three plantations for BA. Precommercial thinning does not appear to exacerbate the incidence of leave-tree mortality from armillaria or heterobasidion root diseases after 30 years, and leave-tree QMD and BA growth increased significantly in most but not all plantations for the tree species sampled. Armillaria and heterobasidion root diseases are not an impediment to precommercial thinning in plantations or stands similar to those we studied.