A total of 602 dogs at the Louisiana State University Veterinary Teaching Hospital were tested for antibodies to Dirofilaria immitis by enzyme-linked immunosorbent assay (ELISA), using a purified adult dirofilaria-derived antigen. Most dogs also were evaluated for heartworm infection by a complete WBC count and a Knott test for circulating microfilariae. The serologic prevalence of heartworm infection was 34.7%; prevalence increased significantly (P = 0.0003) with age up to 8 years and then decreased. Dogs usually kept indoors were significantly (P = 0.005) less likely to be infected, as were dogs given diethylcarbamazine therapy (P = 0.0004). Coat length, sex, breed, and presence of intestinal parasites were not associated (P greater than 0.05) with a positive heartworm ELISA result. The ELISA titers showed a positive relationship with both eosinophil and basophil counts. A total of 99 dogs evaluated radiographically were grouped according to results of the Knott test and radiographic examination as follows: (i) negative Knott test and negative radiographic examination (14 dogs), (ii) negative Knott test and positive radiographic examination (57 dogs), and (iii) positive Knott test and positive radiographic examination (28 dogs). The serologic prevalences of D immitis infection in each of these groups were 35.7%, 56.1%, and 85.7% for groups (i), (ii), and (iii), respectively. The ELISA, when used in conjunction with the Knott test results, record of exposure, clinical signs, laboratory results, and radiographic changes, was found to be useful for studying serologic patterns and identifying risk factors for canine heartworm infection.