This paper examines the effects of information technology (IT) on organizing and union effectiveness in US national unions. Original survey data and data from government and independent sources are combined to model outcomes including membership growth, success in representation elections, and union leaders’ assessments of effectiveness as a product of environmental and organizational characteristics. The results suggest that the practical impact of IT use on organizing outcomes can be quite important. Evidence regarding the impact of IT on overall effectiveness (i.e. organizational or union effectiveness) is more mixed.