Several parasitic species are well known to have carcinogenic properties, namely; Schistosoma hematobium (squamous cell carcinoma of the bladder) and the liver flukes Opisthorchis and Chlonorchis (cholangiocarcinoma). A large number of parasites are known to colonize the gastrointestinal tract. We sought to review the evidence that implicates these parasites in gastrointestinal neoplasia. Schistosoma japonicum, which is endemic primarily in east Asia, has been shown in multiple studies to convey a mildly increased risk of colorectal adenocarcinoma. The data supporting a causative role for Schistosoma mansoni in colorectal or other neoplastic processes are less convincing, limited primarily to small case-control studies and case series. Reports of possible associations between other gastrointestinal parasites (e.g., E. histolytica and A. lumbricoides) and neoplasia may be found in the literature but are limited to individual cases. We conclude that, other than S. japonicum and to a lesser extent S. mansoni, there is little evidence of an association between gastrointestinal parasites and neoplasia.