Methods to assess exposure to environmental tobacco smoke need to be valid and relatively easy to use. We therefore explored the use of a 24-h environmental tobacco smoke exposure-recall diary by comparing data from the 24-h diary with questionnaire responses and levels of salivary cotinine--a biochemical marker of environmental tobacco smoke exposure. A total of 875 nonsmokers at five Rhode Island worksites participated in the study. Twenty-five percent of the participants lived with smokers, and 96% had regular exposure to environmental tobacco smoke at work. Individuals who lived with smokers reported more exposures in the 24-h diary, both outside of work and during work hours, compared with those who had no smokers in their household. The correlation between saliva cotinine concentrations and the exposures recorded in the diary was weak (r = .10). Brief instruments for assessment of environmental tobacco smoke should be viewed cautiously, and use of this 24-h recall diary is not recommended.