In an experiment designed to answer the title question, twelve groups of four men each and forty-eight individuals followed the four basic rules of brainstorming in attacking the same three problems in the same order. Upon completion of the experiment, a table of random numbers was used to divide the forty-eight individual subjects into twelve nominal groups of four men each. The performance of each nominal group was then scored as though its members had actually worked together. The achievement of these nominal groups thus provided a measure of the performance to be expected if group participation neither facilitates nor inhibits creative thinking. When compared with that of the twelve nominal groups, the performance of the twelve real groups was found to be markedly inferior with respect to:​ (a) mean total number of ideas produced;​ (b) mean number of unique ideas produced;​ (c) three different measures which weighted the ideas produced differentially with respect to quality. To the extent that the results of the present experiment can be generalized, it must be concluded that group participation when using brainstorming inhibits creative thinking.