The high rate of early pregnancy in Brazil and particularly in Teresina (21.5%), Piauí State, motivated the current study, the aim of which was to identify levels of objective and perceived knowledge on oral contraceptives, as well as predictive reproductive and socio-demographic variables for high knowledge. A cross-sectional study was performed including 278 teenage mothers (15-19 years), with their childbearing history, admitted to four maternity hospitals in Teresina in 2006. Logistic regression provided the basis for the statistical analysis. Nearly 98% of the adolescent mothers showed low objective and perceived knowledge of oral contraceptives. High parity was the only predictor of increased objective knowledge on oral contraceptives. The adolescents' low level of objective and perceived knowledge on use of oral contraceptives emphasized their susceptibility to risky sexual behavior. The findings emphasize the need for a more interactive approach with adolescents concerning their level of objective and perceived knowledge, to reduce the occurrence and repetition of unwanted teenage pregnancy and its negative consequences for the lives of these young mothers and their children.