The objective of this article is to analyze patients’ awareness of their serostatus prior to an AIDS diagnosis and to identify those who are unaware of their HIV infection. The analysis focuses on adult AIDS cases registered in the R.N.S.P. between January 1, 1994, and June 30, 1995, and diagnosed starting in January 1993. Among the 7,705 cases for which the date of the first positive serology test is recorded, HIV status was unknown prior to an AIDS diagnosis in 1,694 cases (22%). Knowledge of seropositivity prior to an AIDS diagnosis—and thus the use of HIV testing—varies widely by transmission group, as well as by sex, age, region of residence, and socioeconomic status. A multivariate analysis will be conducted at a later date to identify the variables independently associated with knowledge of serological status.