Cotinine, the major metabolite of nicotine, is a useful marker of exposure to tobacco smoke. It can be measured in plasma, urine, or saliva. However, distinguishing between active and passive smoking on the basis of a cotinine measurement may be difficult. In order to evaluate the relationship between saliva cotinine concentration and self-reported tobacco smoke exposure in both active and passive smokers, an English-language literature search using MEDLINE was conducted (1973-1989), and the bibliographies of identified articles were reviewed. Of 43 originally identified articles, only 22 met the criteria for inclusion. Specific information regarding population studied, reported tobacco smoke exposure, method of measurement, and cotinine concentrations was assessed. Passive smokers usually have cotinine concentrations in saliva below 5 ng/ml, but heavy passive exposure can result in levels greater than or equal to 10 ng/ml. Levels between 10 and 100 ng/ml may result from infrequent active smoking or regular active smoking with low nicotine intake. Levels greater than 100 ng/ml are probably the result of regular active smoking. Four categorizations of tobacco smoke exposure are suggested on the basis of saliva cotinine concentrations.