OBJECTIVE To determine the prevalence of secondhand smoke (SHS) exposure among infants and young children who received preventive care at pediatric preventative care clinics associated with an urban public hospital. Cotinine, a metabolite of nicotine, has been used to study SHS exposure in population-based studies of children 3 years of age or older. DESIGN Retrospective study using a convenience sample. SETTING Urban county pediatric primary care clinics in San Francisco, California. PARTICIPANTS A total of 496 infants and children (mean [SD] age, 2.4 [1.9] years). INTERVENTIONS Discarded plasma samples (which were routinely collected for lead screening) were tested, and medical records were reviewed, for SHS exposure. MAIN OUTCOME MEASURE Secondhand smoke exposure based on cotinine plasma level and history of exposure in the medical record. RESULTS Thirteen percent of parents reported that their child was exposed to SHS, yet biochemical testing detected cotinine in 55% of samples, at a geometric mean (SD) of 0.23 (3.55) ng/mL. There were no significant sex or age differences. African American children had much higher mean cotinine levels than did Latino children (multiplicative factor change in cotinine, 6.01 ng/ml [95% Cl, 4.49-8.05 ng/ml] [correction]. CONCLUSION In a city with a low smoking rate (12%) and public smoking bans, we documented 55% exposure among infants and young children, using a plasma biomarker, compared with 13% exposure reported by parents. Because SHS is associated with significant respiratory diseases and parents underreport exposure, routine biochemical screening should be considered as a tool to identify and reduce SHS exposure.