Worldwide, adolescents are at risk for sexually transmitted infections (STI). In The Netherlands, test rates among young heterosexual people are low and knowledge on the behavioral determinants of testing is scarce. In this study, we investigated STI testing in more detail with two independent samples of 16-25 year old vocational school students (n = 756/n = 1302). The aim of this study was to examine risk perceptions in relation to STI testing among lower educated adolescents in order to inform the development of an intervention promoting STI testing. We compared multiple measures of risk perception, fear of testing, self-efficacy for testing, and risk knowledge between groups of adolescents engaging and not engaging in risk behavior. The results show that at least half of the participating students with sexual experience underestimated their susceptibility for STI and showed an optimistic bias. Students with sexual experience also considered STI very severe but lacked self-efficacy for testing. This combination can yield a defensive reaction to the threat of STI that, in turn, results in the underestimation of personal risks. However, self-efficacy did not mediate the relationship between intention to test and perceived relative risk. In conclusion, our findings show underestimation of personal risks by vocational school students, a high perceived severity of STI and low self-efficacy for testing. A low level of knowledge regarding sexual risks and symptoms of STI might have contributed to low risk perceptions. Self-efficacy did not change the relation of intention-to-test to perceived risk.