In the first quarter of 2007, the FASB’s FIN 48 required calendar-year companies to disclose new information about their uncertain tax benefit liabilities (UTBs). These liabilities arise from positions, taken in tax returns to defer or avoid taxes, that are not likely to be sustained if audited by taxing authorities. We argue that data newly disclosed under FIN 48 enable us to obtain proxies for (1) companies’ cumulative prior aggressiveness in taking uncertain tax benefit positions, and (2) companies’ cumulative prior aggressiveness in recording UTB liabilities. We refer to the first construct as ‘tax position aggressiveness,’ and to the second as ‘tax reporting aggressiveness.’ We hypothesize and find that both types of aggressiveness are influenced by (1) proxies for managers’ incentives and opportunities for aggressive behavior, and by (2) proxies for managers’ history of similar aggressive behavior. In addition we find that tax position and tax reporting aggressiveness are positively associated.