Upper GI bleeding
Esophagus

Varices

Carcinoma

Reflux esophagitis

Peptic ulcer

Hiatal hernia

Mallory-Weiss syndrome

Foreign body

Stomach

Peptic ulcer

Curling ulcer

Cushing ulcer

Portal hypertension gastropathy

Gastric erosions

Dieulafoy lesion

Ulcerated gastric leiomyoma

Hiatal hernia

Carcinoma (GI stromal tumor,lymphoma, adenocarcinoma, carcinoid tumors)

Foreign body

Gastritis

Gastric varices

Gastric antral vascular ectasia

Duodenum

Diverticulitis

Polyps

Hemobilia

Peptic ulcer

Vascular malformation

Erosions

Other causes

A-V malformation

Splenic artery aneurysm

Uremia

Anticoagulation

NSAIDs

Aortoenteric fistula

Lower GI bleeding (originates distal to the ligament of Treitz)
Note: Always rule out upper GI bleeding first!
Small intestine

Meckel diverticulum

Intussusception

Mesenteric infarction

Aortoenteric fistula

Tumors

Large intestine

Diverticular disease

A-V malformation

Infectious colitis

Inflammatory bowel disease

Carcinoma

Ischemic colitis

Polyps

Radiation proctitis

Other causes

Endometriosis

CMV inf. (if immunodef.)

Hemorrhoids

Anal fissure

Anal fistula

Anorectal varices