Abstract Exposure of nonsmokers to environmental tobacco smoke (ETS) has become an important public health issue; it is generally agreed that increased exposure is related to morbidity and mortality. Precise prevalence estimates of exposure are not yet available, and measurement methodology for ETS exposure rates is still in its formative stage. Recent interventions have attempted to reduce ETS exposure, particularly in children of smoking parents. Studies have relied primarily upon reduction of parents' smoking rates to indirectly reduce children's ETS exposure. In order to effectively design interventions to achieve reductions in ETS exposure, more attention must be given to smoking behaviors which lead to passive exposure. Ninety-one families with at least one smoking parent and an asthmatic child were recruited from four allergy clinics, and interviewed regarding their smoking history, current residential smoking patterns, and the children's exposure patterns. Descriptive data are presented. It is striking that the most smoking and most exposure did not occur in the same locations, time, or during the same activities. It is recommended that interventions focus closely on these patterns rather than on reduction of smoking rates alone, in order to effect reduction in ETS exposure.