As the knowledge economy gains momentum, professional service firms (PSFs) as one of its key elements are receiving a growing academic attention. This paper provides an empirical test on PSFs’ governance-performance effects with a panel dataset of the US top 100 accounting firms covering the 2003-2012 period, and based on a review of the literature on factors in play for PSFs’ performance. The results show that different measures of performance may result in different governance-performance effects, but on balance the partnership format heightens PSFs’ performance. The article ends with a brief conclusion.