The apparent prevalence of endoparasite infections across different age groups was calculated from 6555 dogs and 1566 cats that had a fecal examination performed upon presentation to the Veterinary Hospital of the University of Pennsylvania between 1997 and 2007. Based on notations from the medical history indicating prior parasite infections, estimates of recurrence were generated for each common group of parasites, including Trichuris, Giardia, ascarids, hookworms, Cystoisospora, and tapeworms. Endoparasitism was predominantly a disease of younger animals, with peak prevalence observed almost uniformly in dogs under 6 months old, with the exception of Trichuris with its longer pre-patent period, and in cats less than 18 months old. Furthermore, nearly 50% of dogs under 6 months old with a history of parasites, were diagnosed with at least one species of parasite on subsequent fecal examination. The percentage dropped to 18.4% in animals aged 1-4 years, but again increased to 31.5% in animals over 10 years old. There was no reported recurrence of Giardia or Cystoisospora from canine or feline patients older than 1 year. The recurrence of whipworm rose steadily with age, while hookworm and roundworm recurrence peaked in patients 1-4 years old. Findings from the study emphasize the importance of follow up fecal examinations and treatments in patients diagnosed with endoparasites.