The authors aimed to evaluate the prevalence of problem behaviors in a representative randomized national sample of youth in the Czech Republic. The Social and Health Assessment (SAHA), a school survey, was administered to a nationwide sample of 4,980 adolescents in three age cohorts (12, 14, and 16 years) in Prague and 12 regional capitals of the Czech Republic. Three antisocial behaviors, i.e. fistfight, damage of public or private property, and shoplifting, were the most frequent in both genders, while carrying a gun or a weapon (blade, knife, gun) to school were also highly prevalent among boys. The levels of alcohol and cigarette use were high, and increased with age. Marijuana, amphetamines and met-amphetamines were the most frequently used illegal drugs in both genders. Risky sexual behaviors, assessed in 16-year-old adolescents, were also common, with only 64.5% of boys and 46.8% of girls reporting condom use and 27.7 of boys and 21.0% of girls reporting that they had 4 or more sexual partners. This study demonstrated a relatively high prevalence of risk behaviors in Czech youth, a finding with major implications for public health and future research.