OBJECTIVE: Refeeding syndrome is a feared complication of refeeding patients with anorexia nervosa. There are now a number of controlled studies showing that refeeding with an initial high calorie count is more beneficial than cautious refeeding and is safe under continuous monitoring. However, there have yet not been studies in severe anorexia nervosa. METHOD: We present an observational study in two different samples. The first sample consists of those 1075 out of a total of 3230 patients with anorexia nervosa treated in our hospital within 4 years for whom a complete admission laboratory was available and who had an age of at least 18 years at admission. A risk score was calculated from the number of pathological laboratory values out of 12 parameters indicating either refeeding syndrome or health hazards related to malnutrition. The second sample was obtained from a special ward for patients with eating disorders medically at-risk. During the period in question, 410 patients with anorexia nervosa were treated there. 142 patients had a BMI of 13 or less and at the same time a complete data set with the mentioned 12 laboratory parameters at admission and weekly in the following 4 weeks after admission. RESULTS: The risk represented by the laboratory parameters is significantly and negatively correlated to BMI and much higher for the group of patients with a BMI below 13 than for those with a higher BMI (χ DISCUSSION: Under thorough medical surveillance, supplementation of phosphate and thiamine, and substitution of electrolytes whenever necessary rapid renutrition appeared to be save even in extremely malnourished inpatients with anorexia nervosa. As measured by the laboratory values, the health status of the severely malnourished patients improves significantly on a high-calorie diet. Except for hyperhydration, there was no evidence of a refeeding syndrome.