Prevalence of cigarette smoking varies widely in different countries, ranging, at the age of 13, from 2% to 5% (Sweden, United States) to more than 30% (Australia, Uruguay). Even if the prevalence of smokers among male adolescents is decreasing in western countries, it is increasing among girls and, in developing countries, male adolescent smokers still reach 40% (and up to 70%-80%). The determination of saliva cotinine levels, a product of nicotine metabolism, is an useful indicator of exposure to passive or active smoking. In a study of 210 children aged 9-13 years, we found detectable levels of saliva cotinine in 13% of children who lived in nonsmoking families and denied being regular smokers and in only 60% of children living in families with heavy cigarette consumption. This wide variations of a marker of smoking exposure may explain the differences in reported consequences of tobacco smoke in investigations conducted in different places and on different target populations. We later studied the effects of environmental tobacco smoke (ETS) in a sample of 166 nine-year-old children. The relationship between parental smoking and degree of bronchial responsiveness in males was significant. Also, prick skin test reactivity to allergens was significantly increased in children of smoking parents. Many studies concerning the effects of ETS exposure seem to demonstrate that not only the lungs and not only children are affected even if the difficulty of the epidemiology in such a field must suggest caution in interpreting the results.(ABSTRACT TRUNCATED AT 250 WORDS)