An endoscopic study of 89 patients with an oligosymptomatic course of schistosomiasis (Manson's schistosomiasis 63 patients, urinary schistosomiasis 26, aged 17-28 years) from endemic foci, was carried out. In patients with Manson's schistosomiasis the mucous membrane was examined by rectoromanoscopy and colonofibroscopy, and in patients with urinary schistosomiasis by cystoscopy. Endoscopy was used with biopsy and histology of the mucous membrane of the injured organs. In intestinal schistosomiasis pathologic changes of the mucous membrane were observed in 88.8% of the patients, in urinary schistosomiasis in 84.6% of cases. In spite of the oligosymptomatic course of the diseases, severe complications were found: liver cirrhosis with portal hypertension in 2 patients with intestinal schistosomiasis (revealed by laparoscopy), and cancer of the urinary bladder in patients with urinary schistosomiasis (revealed by the histologic study of biopsy material). The histologic study of biopsy demonstrated the signs of chronic productive inflammation of all the patients examined.