Afferent loop syndrome (ALS) is a complication of partial gastrectomy, gastrojejunostomy, and Roux-en-Y reconstruction surgery where the afferent loop of the small bowel is completely or partially obstructed. This can manifest as an acute or chronic syndrome. ALS is characterized by abdominal pain, diarrhea, nausea, and severe bilious vomiting. It can be associated with cholangitis, malabsorption syndrome, and intestinal bacterial overgrowth. Life-threatening acute onset ALS usually follows shortly after surgery and can manifest as complete obstruction, a serious medical emergency. Onset of chronic ALS may occur years after surgery.

It can be prevented by careful choice of appropriate gastric or abdominal surgical technique. Following surgery, when complete obstruction occurs due to afferent loop, the type of corrective surgery depends on the cause and is of immediate urgency.