The afferent loop syndromes result from obstruction to the afferent jejunal loop. Acute ALS results from complete obstruction, usually occurs early after surgery and runs a devastatingly lethal course unless promptly treated by reoperation. In chronic ALS the obstruction is intermittent and produces a clinical syndrome from which a diagnostic histroy can usually be obtained. Although the exact incidence is unknown, it is certainly not rare, especially in antecolic Billroth II gastrectomies. Treatment consists of doing away with the afferent loop. In gastroenterostomy alone takedown of the anastomosis with a Weinberg pyloroplasty is the treatment of choice. The safest and simplest treatment for patients whose original operation was Billroth II gastrectomy is conversion to a Roux-en-Y procedure. In all cases vagotomy should be added unless previously performed. No medical treatment is available and patients with no other contraindication should have revisional surgery if symptoms are clinically significant. Both acute and chronic afferent loop syndromes should be completely prevented by appropriate choice of the initial operative procedure. The vagotomized stomach should be drained by pyloroplasty, not gastrojejunostomy. Vagotomy and antrectomy should be reconstructed with a Billroth I gastroduodenostomy. The Braun enteroanastomosis should be utilized after subtotal gastrectomy for carcinoma. The wider application of parietal cell vagotomy for duodenal ulcer deserves close observation and further consideration.