This work addresses the relevance of establishing a Brazilian database consisting of public domain patent documents through the use of the International Patent Classification (IPC). Between 1971 and 2001, a total of 245,225 patent applications have been filed with the Brazilian Patent Office and of those 130,566 (53.24%) were found to be in the public domain. Public domain patents are defined here as extinct patents as well as patent applications, which are: Applications on which the request for examination of the application has not been made; Applications under examination, on which requirements made by the examiner were not answered; Rejected applications; Patents whose annuity has not been paid. A subset from this public domain set-34,429 patent documents from 1992 to 1995-were analyzed. From these, 40.50% (13,945) were found to be in the public domain. As a result, it was found that, at the broadest level, the highest incidence technological areas for Brazilian public domain patent documents were "Human Necessities" (IPC Section A) and "Chemistry and Metallurgy" (IPC Section C). It is postulated that these areas are of extreme importance for the country. More deatiled analyses are also presented, at class, subclass, and subgroup levels.