Smoking prevention efforts should either prevent target groups from becoming susceptible to smoking or prevent susceptible adolescents from progressing to becoming regular smokers. To describe the prevalence of susceptibility to smoking among never smoker students from cities that applied the GYTS in 2003 and 2006. The GYTS uses a two-stage cluster sample survey design that produces representative samples of students aged 12-15 years enrolled in public, private, and technical schools. The survey was undertaken at 399 schools in 9 cities. The GYTS surveyed 33,297 students during the academic years 2003-04 and 2006-07. Among never smokers, about 25% are likely to initiate smoking in the next 12 months. There are no differences in susceptibility to smoking by gender. When comparing results from 2003 and 2006, the susceptibility index has not changed, but for one city. The GYTS results are useful for monitoring susceptibility to smoking among adolescents and provide evidence for strengthening the efforts of tobacco control programs in Mexico.