Canine heartworm (Dirofilaria immitis) disease affects wild canids and may be a factor impacting the health and population dynamics of coyotes (Canis latrans). Coyotes may serve also as a potential reservoir for transmission of these parasites to domestic dogs. We investigated 920 coyotes harvested by hunters and trappers throughout Illinois (USA) from 1995–1997. The objectives of the study were to: 1) survey the regional prevalence and intensity of heartworms in coyotes in Illinois, 2) determine whether heartworm intensity correlates with physical condition, particularly body weight and winter fat levels, and 3) evaluate the relationship between heartworm infections and the reproductive success of females. Prevalence of heartworms statewide was 16.0%. Prevalence was significantly higher in males (17.7%) than in females (14.1%; P=0.04) and was higher in the older age-classes (P<0.0001). The regional prevalence of heartworms increased from northern to southern Illinois. Intensity ranged from 1 to 111 with a mean of 8.7 (SD=13.2) worms. Intensities did not differ significantly between sexes (P=0.53) or among age-classes (P=0.84). Most infected coyotes had low intensity infections, 78.2% carried <12 heartworms, 11.6% had 12–24 worms, and 10.2% were infected with >24 worms. Body weights were not correlated with the presence of heartworms, nor were levels of kidney fat and marrow fat. However, reproductive success was lower in infected females. The percent of yearling females that bred was lower among infected females, as was the number of offspring produced by adults ≥3.5 yr old. Our study demonstrates that heavy infections adversely affect fur quality and reduce fecundity of some females, but these effects are small and few coyotes (4.1%) had enough worms to trigger them. Coyote populations have increased in Illinois during the past 20 yr, but prevalence and intensity of heartworm disease appears to have changed little in that period. We conclude that heartworm disease is only a minor factor influencing coyote population dynamics in Illinois.