Recently a novel and non-invasive technique of lower leg length measurement (knemometry) was introduced. The method estimates the distance between heel and knee in the sitting child with an accuracy of 0.09 mm(SD). Two female patients with anorexia nervosa, aged 14:4 and 13:7 years, weighing minus 27 and minus 38% of normal weight for age, were measured repeatedly with this method within periods of 72 and 129 days during hospitalization und up to 133 days thereafter. During the first 40 days of hospitalization, patient 1 was measured 30 times and showed significant shrinkage of the lower leg length of -0.040 mm/day (P less than 0.001). Over the remainder of her hospital stay her mean lower leg length increment still was much less (+0.036 mm/day) than that expected just by increase of soft tissue due to rapid weight gain (+0.126 kg/day). The second patient also showed significant shrinkage of lower leg length during a 4 month period after discharge from the hospital (-0.009 mm/day, P less than 0.01). This is the first demonstration of long term lower leg shrinkage in children. Its origin remains unclear, but this shrinkage is clearly distinct from either arbitrary changes of lower leg length due to errors of measurement or changes of body diameters due to temporary alterations of soft tissue (i.e. during periods of weight loss). We speculate that progressive atrophic changes of the epiphyseal plates still open in both patients may account for the observed long term shrinkage of lower leg length.