Hiatal hernia is a condition in which part of the stomach sticks upward into the chest, through an opening in the diaphragm. The diaphragm is used in breathing; it is the sheet of muscle that separates the chest from the abdomen. Hiatal hernias may be due to a weakening of the supporting tissue. Increasing age, obesity, and smoking are known risk factors in adults. Hiatal hernias are very common, especially in people over 50 years old. This condition may cause reflux (backflow) of gastric acid from the stomach into the esophagus.