Epidemiologic studies of companion animals such as dogs have been established as models for the relationship between exposure to environmental tobacco smoke (ETS) and cancer risk in humans. While results from these studies are provocative, pet owner report of a dog's ETS exposure has not yet been validated. We have evaluated the relationship between dog owner's report of household smoking by questionnaire and dog's urinary cotinine level. Between January and October 2005, dog owners presenting their pet for non-emergency veterinary care at the Foster Hospital for Small Animals at Cummings School of Veterinary Medicine, Tufts University, were asked to complete a 10-page questionnaire measuring exposure to household ETS in the previous 24 h and other factors. A free-catch urine sample was also collected from dogs. Urinary cotinine level was assayed for 63 dogs, including 30 whose owners reported household smoking and 33 unexposed dogs matched on age and month of enrollment. Urinary cotinine level was significantly higher in dogs exposed to household smoking in the 24 h before urine collection compared to unexposed dogs (14.6 ng/ml vs. 7.4 ng/ml; P=0.02). After adjustment for other factors, cotinine level increased linearly with number of cigarettes smoked by all household members (P=0.004). Other canine characteristics including age, body composition and nose length were also associated with cotinine level. Findings from our study suggest that household smoking levels as assessed by questionnaire are significantly associated with canine cotinine levels.