Ten patients with fatty liver changes were subjected to liver biopsies. In seven, ultrasonography showed focal hypoechogenicity within a "bright" liver, generally interpreted as focal sparing. Three patients had hyperechoic areas surrounded by "normal" parenchyma usually felt to represent local fat accumulation without diffuse involvement. Morphometric analysis was used to determine the total area occupied by fat (area density) and the number of lipid droplets (numerical density) per microscopic field in lesional and perilesional specimens. Compared with respective perilesional values, hypoechoic lesional numerical densities were significantly lower in 6 of 7 patients; the three hyperechoic samples had significantly higher values. In most cases, lesional and perilesional area densities were not significantly different. These hypoechoic focal lesions are believed to be merely areas in which a similar quantity of fat is contained in fewer droplets, and focal hyperechogenicity is believed to result from a larger number of fat-filled vacuoles with respect to that of the surrounding parenchyma. Variation in the number of solid-liquid interfaces causes the ultrasonic contrast between these lesions and the similarity fatty liver parenchyma surrounding them.