This qualitative narrative study examined contraceptive use and non-use in light of the Theory of Planned Behaviour. The purpose of this paper was to understand contraceptive use and non-use among Finnish teenage girls: why do girls use or not use contraception in a sexually motivated situation and how do the determinants of the Theory of Planned Behaviour (intention, attitudes, social norms, self-efficacy and preparatory behaviour) and situational factors affect girls' contraception-related behaviour? The data are comprised of the narratives of 41 girls regarding their experience with sexually motivated situations that involved contraceptive use or non-use. The narratives were analysed with narrative analysis, a categorical-content mode of reading. The findings suggest that, in light of the Theory of Planned Behaviour and its key determinants, attitude, intention and preparatory behaviour, like carrying condoms, having a condom available and discussing safer sex before intercourse, were not alone sufficient for predicting contraceptive use. The crucial element seems to be self-efficacy. This study provides situational information about adolescent girls' contraception-related decision-making and practical implications for health promoters and sex educators. (Contains 3 tables.)