A case-control study was conducted to investigate the environmental causes of sinonasal cancers among pet dogs. Sinonasal cancer (SNC) cases and digestive cancer controls from the years 1989 through 1993 were obtained from a veterinary histopathology database. Owners were mailed a self-administered survey requesting information on canine factors, owner demographics, household exposures (including environmental tobacco smoke), and local pollution. A total of 129 case owners and 176 control owners returned completed surveys: a response rate of approximately 72% . Only household exposures were associated with increased SNC risk. Use of indoor coal or kerosene heaters represented the strongest risk factors, with significant adjusted odds ratios of 4.2 and 2.2 respectively. Environmental tobacco smoke exposure was not a risk factor and was suggestive of a nonsignificant, mildly protective effect at the lower exposure levels. Increasing nasal length was a significant risk factor, and there was effect modification between nasal length and coal or kerosene combustion. No self-reported measures of local pollution, such as urban status or residence within 1 mile of a factory, were associated with SNC risk. These results suggest that canine SNC has a strong environmental component and highlight the importance of indoor exposures, especially from fossil fuel combustion products. These results also suggest that pet dogs represent excellent sentinels for indoor cancer risk and that canine SNC cases can be used as early markers of household exposure to carcinogenic combustion products.