An observation of late visceral syphilis in a senile patient complicated by syphilitic abdominal aortic aneurysm with aortoduodenal fistula and fatal intestinal bleeding is presented. The patient was taken to the hospital by an ambulance with complaints of severe weakness, dizziness, and black loose stool in the last few days. The patient was hospitalized in the intensive care unit. A history of gastrointestinal bleeding diagnosed several months ago. Repeated esophagogastroduodenoscopy and colonoscopy revealed twice-resuming gastrointestinal bleeding during 5 days of hospital stay. The source of bleeding has not been found. CT made it possible to diagnose aneurysm (mycotic?) of the infrarenal section of the abdominal aorta and suggest the presence of an aortoduodenal fistula. On the 5th day of hospitalization, the patient died. Syphilis is diagnosed pathomorphologically posthumously. Syphilitic gummas were found in the walls of the aorta, duodenum, and adjacent retroperitoneal tissue, and productive and granulomatous inflammation was found in the liver, lungs, heart, and pia mater. The diagnosis of tertiary visceral syphilis was confirmed histologically, including the finding of pale treponema during staining by Levaditi. The aortoduodenal fistula was formed by the fusion and decay of the gummas of the adjacent to each other the walls of the aorta and duodenum. During the life of the patient, serological tests for syphilis were not carried out.