Compositions for forming a silicone elastomer and the silicone elastomers thereby which have improved characteristics and which can be used for medical devices are disclosed. The composition for forming a silicone elastomer can include (A) an organopolysiloxane having silicon-bonded alkenyl groups； (B) an organohydrogensiloxane having an average of two or more silicon-bonded hydrogen atoms in the molecule； (C) an inorganic filler； and (D) a filler treatment agent which includes an alkenyl-containing group, e.g., an alkenyl-containing organosilane or organosilazane.
