The positive rate of house-dogs for Dirofilaria immitis microfilariae decreased in 27 years from 1968 to 1994 in northern and southern parts of Nagasaki City. It was examined whether or not the increase of indoor-kept dogs contributed greatly to the reduction in positive rate of house-dogs in the city. Positive rate of dogs was generally lower in indoor-kept dogs than in outdoor-kept dogs. From the lower positive rate of indoor-kept dogs, D. immitis infection is considered to have occurred mainly in outdoor-kept dogs. Positive rate was high from 1968 to 1983 when rate of indoor-kept dogs was low and thereafter positive rate gradually decreased though the rate of indoor-kept dogs was not greatly changed. From this, increase of indoor-kept dogs may be a factor causing the decrease of positive rate in all examined dogs from 1968 to 1983, but the reduction of positive rate thereafter is considered to be due to decrease of positive rate in outdoor-kept dogs.
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