The first report of canine Lyme borreliosis was published in 1984, and since that time there have been numerous reports. Clinical manifestations have varied, and serology is often used to make the diagnosis. This review is designed to inform veterinary clinicians about the current serologic tests and their limitations. In endemic areas, the fact that up to 50% of the canine population can be asymptomatic, yet seropositive, suggests that positive antibody titers do not necessarily indicate the presence of disease. Instead, the presence of a positive antibody titer should be considered evidence of exposure to the agent. Many dogs apparently are exposed to the agent of Lyme borreliosis but do not come down with the disease. Immunoblots are not able to distinguish between infection versus exposure. Newer diagnostic tests, such as antigen assays, are being evaluated. However, the fact that spirochetes have been isolated from 8% of asymptomatic dogs living in an endemic area is evidence that these tests will also need to be cautiously interpreted. Our inability to characterize the relationship between clinical signs and serology has hampered our understanding of canine Lyme disease. Presently, the diagnosis should be one of exclusion. A diagnosis of Lyme borreliosis should be considered confirmed only when other diseases are completely ruled out.