OBJECTIVE: to analyze the sexual behavior and biodemography factors associated with sexually transmitted infections (STIs). METHODS: case-control study conducted at the San Jose de Coronel Hospital, Chile, in 2007. In a sample of 100 women between 15 and 34 years old, 50 with STIs and 50 without STIs, a self-administered instrument was applied for sociocultural and sexual behaviour variables. The 2 and Fisher's tests were used. RESULTS: there were statistically significant differences at the age of initiating sexual relations, the number of sexual partners, the use of condom, and the knowledge about the contagious nature of STIs. Major infections were acuminated condyloma (HPV) 52% and syphilis (25%). CONCLUSI ONS: the risk sexual behavior and the poor information about the infection are factors associated with sexually transmitted infections.