This article sets out to study the dimensionality of adolescents' knowledge of HIV transmission by contrasting a two-factor and a one-factor model using confirmatory factor analysis (LISREL). The empirical data stem from a longitudinal study on health behaviour in Norway. This study uses cross-sectional data of the same subjects in 1990 with mean age 13.3 (n = 927) and 1992 ( n = 974). The respondents were confronted with 12 items describing different types of risky situations and asked whether HIV could be transmitted in these situations. The two-factor model provided a significantly better fit to the empirical data than did the one-factor model, suggesting that knowledge of how HIV is transmitted is distinct from knowledge of how HIV is not transmitted. Several explanations for this difference are suggested, and implications for constructing health- education messages are discussed.