Black locust is one of the most important stand forming tree species in Hungary, covering approximately 23% of the forested land (410 thousand ha) and providing 23% of the annual timber cut of the country. Hence, during the past two decades several experiments with this species have been carried out in the country. This paper investigates the influence of thinning on the diameter increment in an experiment including four plots. One plot was left as an unthinned control plot and three plots were thinned with various thinning intensities at age 22. The experiment was measured at ages 22, 27, 32, and 36. Apparently the thinning intensities applied on two of the plots did not come up to the intensity required to influence the diameter growth of the remaining upper-storey trees. Therefore, for a given tree size the growth of the trees on these two plots did not differ significantly from the growth of similar trees in the unthinned plot. By contrast, significant thinning effects on the diameter increment of individual trees were observed on the most heavily thinned plot. Here the diameter increment was enhanced for trees with diameters of less than 20 cm, whereas the largest trees of the stand exhibited moderately increased growth. Apparently a stand density corresponding to the heavy thinning treatment (Fw values of 22-23) is required to get a significant thinning response. On the other hand, to avoid thinning shocks, it may be recommendable to apply two moderate thinning treatments instead of a single heavy one.