The serum and urinary levels of electrolytes were measured in 25 patients with anorexia nervosa admitted to this hospital. Seven (28%) of these patients vomited, usually surreptitiously. Hypokalemia was detected in 5 (20%) of these patients, all among those who vomited. The urinary chloride/sodium ratios were low in the patients who vomited, and did not overlap the values in patients who did not vomit, indicating that this ratio was a good indication of vomiting. These results suggested that stopping vomiting is most important for correcting hypokalemia.