We performed a survey to assess the exposure to environmental tobacco smoke (ETS) in 1-year-old infants in Thailand. Of the 725 infants, it was reported that 73.3% had household smoking and 40.7% had detectable urinary cotinine. Twenty-five infants (3.4%) had urinary cotinine in the range of adult heavy smokers. The prevalence of ETS exposure was significantly higher in infants with a father whose education was < or = grade 6 than in those with father's education >6 years (44.0% vs. 36.0%, p = 0.039). Data on the exposure to ETS among infants will provide prevalence information and identify population subgroups at increased risk for exposure.