The presence and localization of fat in human lung tissue was evaluated by the "en bloc" staining procedure with osmium tetroxide performed with formalin fixed tissue with subsequent paraffin sectioning and with the oil red O technique performed with frozen sections. Fifty-one autopsy cases were divided into three clinical groups: group A, consisting of 17 patients with severe skeletal trauma; group B, consisting of 14 patients with minor skeletal trauma; and group C, consisting of 20 control patients without trauma. Adjacent sections of lung were selected from all cases, stained with the "en bloc" osmium tetroxide and oil red O methods, coded, and examined under the light microscope without knowledge of the clinical grouping, Stainable fat was graded on a 1 to 4+ scale, and attention was given to histologic localization in tissue sections. The "en bloc" osmium tetroxide technique revealed greater amounts of stainable lipid in clinical groups A and B and was most effective in demonstrating lipid when present in small quantities (group B). Since the method permits the employment of paraffin sections, evaluation of fine histologic detail is an advantage that is not always obtained in frozen sections. It is concluded that the "en bloc" osmium tetroxide technique is superior to the oil red O technique in terms of visualization and histologic localization of minute amounts of lipid in lung tissue.