One of the most important problems in the evolution of legacy systems is the loss of knowledge about them. In this paper, we present an approach for extracting structural business rules from legacy databases. We used the technique to recover business rules from SIFI (SIstema Fiduciario Integra do), an existing legacy system, implemented mostly in PL/SQL and Oracle Forms. Four employees of the company that know the system and its domain evaluated the extracted rules in order to assess the precision of the extraction technique. The results show that 29% of recovered rules are correct structural business rules, 36% correspond to implementation rules, and 35% are incomplete or incorrect rules. The recovery technique proves to be practical, while there is room for improvement, and it will be used as basis for the recovery of additional types of business rules.