Blastomycosis occurred in six patients in five households. In each instance one or more dogs living with the family or living near the family also developed blastomycosis. The recognition of canine blastomycosis helped in the early diagnosis of human cases. Because both dogs and patients were probably infected at the same place, canine blastomycosis may be an important epidemiologic marker, alerting physicians to the possible presence of concomitant blastomycosis in humans.