We collected fecal samples from 32 free-ranging, 19 semicaptive, and 54 captive Sumatran orangutans on Sumatra from 1998 until 2004 and screened them for gastrointestinal parasites. Our objectives were to compare the intestinal parasites of free-ranging, semicaptive, and captive orangutans and to evaluate the risk of parasite transmission in orangutan reintroduction programs. We identified 4 genera of Protozoa, 7 genera of nematodes, 1 trematode sp., and 1 cestode sp. The prevalence of Balantidium coli in free-ranging orangutans was significantly higher than in captive individuals. However, the prevalence of Strongyloides sp. was higher in captive than in free-ranging orangutans. Free-ranging female orangutans had a significantly higher total prevalence of intestinal parasites than that of males. We found no significant difference between parasite prevalences in different age groups. Compared to gorillas and chimpanzees, orangutans carry a smaller variety of protozoan and nematode species. Strongyloides sp. infections form the highest risk in reintroduction programs as crowding, ground-dwelling, and poor hygiene in captive and semicaptive orangutans may cause a constant reinfection.