The authors evaluated the effectiveness of a school-based sex education program in decreasing rates of sexual intercourse, improving birth control use, and decreasing the incidence of pregnancies among teenagers 16 years of age and younger. Twenty-one schools received either the McMaster Teen Program or the conventional didactic sex education program. Preprogram, the mean age of the students was 12.6 years. There were no statistically significant differences between groups in time to first sexual activity for males, χ2(1) = 2.93, p = 0.09; time to first sexual activity for females, χ2(1) = 0.50, p = 0.48; and time to first pregnancy, χ2(1) = 1.90, p = 0.17. Significantly more experimental group males reported always using birth control at year 1 (difference 8.9%; 95% confidence interval [CI] = 0.4,17.4). Limitations of the program that may have influenced the results were the exclusion of contraception information and its short duration.