The epidemiologic features of an outbreak of canine distemper during 1992 and 1993 in pet dogs in Indiana are described. Risk factors for disease were characterized using hospital records of private veterinary practitioners. The risk of disease for purebred dogs was 85% lower than the risk of disease for mixed-breed dogs. Lack of vaccination was associated with a 350-fold increase in the risk of canine distemper, and 93.8% of all cases could be attributed to the lack of vaccination. For many of the owners, the cost of medical treatment exceeded the cost of a vaccination program.