The effects of the pine needle gall midge, Thecodiplosis japonensis, on the growth of the Korean red pine, Pinus densiflora, were examined for the natural forest stands at nine locations: Seocheon-gun, Boryong-gun, and Hongseong-gun in Chooncheongnam-do, where consequence of the infestation had been different. To estimate the economic threshold level of the pine needle gall midge, the growth impacts due to the insect were examined with terminal shoot growth, and cambial growth by analyzing the growth horizontally and vertically. The results obtained are as follows: 1) In general, the growth impacts of the red pine trees were significant for the five consecutive years since damage had occurred, and its maximum retardation was shown in the second and the third years. 2) Simultaneous retardation occurred in both terminal and cambial growth of the tree, probably due to cumulative characteristics of the feeding behaviour. 3) Strong correlation was found between terminal and cambial growth of Korean red pine and this was true for the trees infested by pine needle gall midge, Thus retardation in cambial growth due to the insect may be estimated with that in terminal growth. 4) Infestation was greater on slope facing north than south even though the history of the infestation had been similar. 5) The stem volume of 13-year-old, infested Korean red pine was reduced by more than 75-85%, compared to same aged, uninfected pine in yield table. Thus, much longer period is required to recover the normal growth.