Increasing knowledge is an important goal of human immunodeficiency virus (HIV) prevention strategies, although increased knowledge may not be associated with increased preventive behaviors. This study examines the association of (1) objective and subjective acquired immunodeficiency syndrome (AIDS) knowledge, and (2) both objective and subjective AIDS knowledge with HIV testing use. Data are from the 1988 National Health Interview Survey. Objective and subjective knowledge were only moderately correlated. In regression analyses, higher subjective knowledge was significantly associated with higher testing use, but objective knowledge was not. The results are relevant to other preventive behaviors for which knowledge is an important factor.