Girls take up smoking at least as frequently as boys. Few studies have focused on gender differences in the impact of adolescent smoking. We evaluated the sex-specific effect of adolescent smoking on respiratory symptoms and lung function. All students in junior high and high schools in Nord-Trøndelag County, Norway, 1995–97, were invited to participate in a cross-sectional study. Information on smoking habits and respiratory symptoms was obtained by self-administered questionnaires. Spirometry was performed in accordance with ATS standards. Eight-thousand-three-hundred and five students (83%) completed both questionnaire and spirometry. Among 6811 students aged 13–18 years (50.3% girls) with no history of asthma, 2993 (43.9%) reported never smoking, 665 (9.8%) reported occasional smoking, and 667 (9.9%) reported daily smoking (mean initiation age: 13.9 years). More boys than girls were heavy smokers. In all smoking categories, smokers reported a higher prevalence of respiratory symptoms than nonsmokers; symptoms increased with smoke burden. Girls reported more symptoms compared to boys with comparable smoke burden. A dose–response relation between smoking and reduced lung function was found only in girls. Girls were more vulnerable than boys to the impact of smoking on respiratory symptoms and lung function.