1. A chronic alcoholic with severe metabolic acidosis presents a difficult diagnostic problem. The most common cause is alcoholic ketoacidosis, a syndrome with a typical history but often misleading laboratory findings. This paper will focus on this important and probably underdiagnosed syndrome. 2. The disorder occurs in alcoholics who have had a heavy drinking-bout culminating in severe vomiting, with resulting dehydration, starvation, and then a beta-hydroxybutyrate dominated ketoacidosis. 3. Awareness of this syndrome, thorough history-taking, physical examination and routine laboratory analyses will usually lead to a correct diagnosis. 4. The treatment is simply replacement of fluid, glucose, electrolytes and thiamine. Insulin or alkali should be avoided. 5. The most important differential diagnoses are diabetic ketoacidosis, lactic acidosis and salicylate, methanol or ethylene glycol poisoning, conditions which require quite different treatment. 6. The diagnostic management of unclear cases should always include toxicological tests, urine microscopy for calcium oxalate crystals and calculation of the serum anion and osmolal gaps. 7. It is suggested here, however, that the value of the osmolal gap should be considered against a higher reference limit than has previously been recommended. An osmolal gap above 25 mosm/kg, in a patient with an increased anion gap acidosis, is a strong indicator of methanol or ethylene glycol intoxication.