A previously described experimental hematoma model, achieved by the subcutaneous injection of 12 ml of autologous hemolyzed blood clotted in situ, was made in 33 rats. Sventeen animals served as controls; the other 16 received daily intra-muscular injections of dexamethasone. After an initial decrease in size, 47% of the lesions in the control animals enlarged to a mean weight of 12.1 +/- 2.5 gm, while the lesions in the 16 steroid-treated rats weighted 3.2 +/- 0.70 gm (p less than 0.01). Histologically, lesions from the steroid group showed absence of neomembrane formation. These data offer further support to the theory that the neomembrane development and subsequent enlargement of subdural hematomas is due to inflammatory reactions of tissues in contact with large blood clots.