Using a 1-stage random-digit dial telephone survey, we estimated the number of pet dogs and cats and cancer case ascertainment in the principal catchment area of an animal tumor registry in Indiana, the Purdue Comparative Oncology Program (PCOP). These findings will assist in the estimation of pet cancer incidence rates for the PCOP. The estimated canine and feline populations for Marion County were 144,039 (95% confidence interval, 121,555 to 166,523) and 94,998 (74,384 to 115,648), respectively. For Tippecanoe County (excluding university housing residences), the estimated canine population was 18,000 (14,445 to 21,555), whereas the estimated feline population was 17,165 (12,569 to 21,761). The estimated cancer case ascertainment was 88.3% (dogs, 92.5%; cats, 83.0%) with no statistically significant difference in the estimated ascertainment by county of residence or by species of pet. The amount that owners report themselves willing to pay for treatment of cancer in dogs or cats, however, differed in counties polled. This method's appropriateness for estimating pet populations in general and the validity of the data gathered were supported by response rate of 88.0% and by concurrence with census data for household characteristics previously documented to be associated with pet dog and cat ownership.