This study describes the prevalence and patterns of environmental tobacco smoke (ETS) exposure in a large, well-defined cohort of professional, female school employees in California. Design. This is a cross-sectional study based on survey responses from members of the California Teachers Study (CTS) cohort. Subjects. The analyses focused on lifetime nonsmokers (N = 61,899) in the CTS cohort who responded to detailed questions on lifetime ETS exposures in the home, workplace, and other social settings. Measures. Demographic characteristics, smoking status, and ETS exposure were based on self-reported data from two mailed surveys. Prevalence estimates within the cohort were compared with those from the California Behavioral Risk Factor Survey and the California Adult Tobacco Survey. Results. ETS exposures were highest for never smokers born in the 1930s (78% in the home, 66% in the workplace, and 48% in other social settings) and steadily declined among participants born in later years. ETS exposure from spousal smoking peaked during the 1950s (37%). In the 1980s, the workplace (28%) replaced the household (19%) as the primary exposure setting. Conclusions. Consideration of these patterns in the prevalence of ETS exposures is important in the interpretation and design of tobacco-related health studies. (Am J Health Promot 2004;18[5]:358-365.)