Smoking habits and smoke intake were studied over three consecutive years in 197 girls, initially aged 11 to 14 years. Saliva cotinine concentrations in girls who were smokers throughout the three years increased over each year of the study, the greatest increase occurring during movement from occasional to daily smoking. Cigarette consumption also increased over the two years. For a group of continuing daily smokers (n = 23), inhalation of smoke per cigarette as indexed by the ratio of cotinine concentration to average daily cigarette consumption did not change over time. Cotinine concentrations in 16 girls who were smoking on a daily basis within a year of starting to smoke suggested the early development of inhalation. Our findings suggest that young smokers learn to inhale cigarette smoke very early in their smoking careers, that further increases in smoke intake mainly reflect increased cigarette consumption, and that the pharmacological effects of nicotine are already important in reinforcing their smoking.