The most severe clinical form of American tegumentary leishmaniasis (ATL) due to Leishmania braziliensis is mucosal leishmaniasis (ML), characterized by destructive lesions in the facial mucosa. We performed a retrospective cohort study of 109 ATL patients from Rio de Janeiro State, Brazil, where ATL is caused by L. braziliensis, to evaluate the influence of intestinal parasite coinfections in the clinical course of ATL. Parasitological stool examination (PSE) was performed with samples from all patients by the sedimentation, Kato-Katz and Baermann-Moraes methods. The diagnosis of ATL was made from lesion biopsies by direct observation of amastigotes in Giemsa-stained imprints, isolation of Leishmania promastigotes or histopathological examination.All patients were treated with meglumine antimoniate. Patients with positive PSE had a frequency of mucosal lesions significantly higher than those with negative PSE (p < 0.005). The same was observed for infections with helminths in general (p < 0.05), with nematodes (p < 0.05) and with Ascaris lumbricoides (p < 0.05), but not for protozoan infections. Patients with intestinal parasites had poor response to therapy (therapeutic failure or relapse) significantly more frequently than the patients with negative stool examination (p < 0.005). A similar difference (p < 0.005) was observed between patients with positive and negative results for intestinal helminths, but not for intestinal protozoa. Patients with positive PSE took significantly longer to heal than those with negative PSE (p < 0.005). A similar difference was observed for intestinal helminth infections (p < 0.005), but not for protozoan infections. Our results indicate a deleterious influence of intestinal helminth infections in the clinical course of ATL and evidence for the first time an association between ML and these coinfections, particularly with nematodes and A. lumbricoides.