One explanation proposed for the widespread failure to control zoonotic visceral leishmaniasis by culling infected domestic dogs is that wild canids or humans play significant roles in transmission. The aim of this study was to determine the importance of domestic dogs as the reservoir hosts of visceral leishmaniasis in northwest Iran. A random sample of 3,872 children and 199 dogs in 38 villages was surveyed by the direct agglutination test. Dog ownership details among these households were collected by questionnaire. Parasites isolated from 16 patients and 12 dogs were characterized as Leishmania infantum MON-1. Average seroprevalence in dogs (21.6%) was much higher than in children (7%). Child seropositivity increased significantly with village dog density in absolute terms (P < 0.001) and in relation to dog/human ratios (P = 0.028). Dog ownership within villages also was a significant risk factor for child seropositivity (P = 0.003).