Cat exposure has been directly associated with the development of human Bartonella henselae infections, resulting in cat-scratch disease, bacillary angiomatosis, or bacteremia. The prevalence of serum antibody titers to B. henselae was determined for selected pet cats from 33 geographic locations throughout the United States and several areas in western Canada. Seroprevalences paralleled increasing climatic warmth (P < .02) and annual precipitation (P < .03). These warm, humid areas with the highest seroprevalence would also have the highest number of potential arthropod vectors. The southeastern United States, Hawaii, coastal California, the Pacific Northwest, and the south central plains had the highest average prevalences (54.6%, 47.4%, 40.0%, 34.3%, and 36.7%, respectively). Alaska, the Rocky Mountain-Great Plains region, and the Midwest had low average prevalences (5.0%, 3.7%, and 6.7%, respectively). Overall, 27.9% (175/628) of the cats tested were seropositive. The seroprevalence of B. henselae in cats varies throughout the United States and appears to be influenced by climate.