We report a substantial prevalence study in symptomatic pet dogs of important zoonotic parasitic enteric infections. A total of 4526 dogs which had a faecal sample submitted to a diagnostic laboratory in the UK between 2003 and 2005 were included in the study. The most common parasite was Giardia spp., which was found in 380/4526 dogs (8.4%, 95% CI 7.6-9.2%). Surprisingly, Cryptosporidium spp. infection was detected in only 29/4526 (0.6%, 95% CI 0.4-0.9%). Toxocara canis was found in 63/4526 dogs (1.4%; 95% CI 1.1-1.8%). Prevalence of Giardia (P < 0.001) was significantly higher in dogs <12 months of age, with nearly one-fifth of all symptomatic dogs under 6 months being infected with Giardia. Some seasonality was detected with a higher prevalence of Cryptosporidium oocyst shedding found from October to December. These data are of importance for veterinarians in judging the likelihood of enteric parasitic infection in an individual with clinical signs. Moreover, they provide information to direct future work in determining the risk to the human population from parasitic zoonoses of dogs.