According to Wikipedia, the USSR had over 8 million casualties. This was over 10 times the amount suffered by the Americans and British combined. (Although I'm not sure how many of those casualties happened prior to Germany's attack.)
Well, Soviets suffered almost 3.2 million irrecoverable casualties in the first 5 months of the war, more than a quarter of the total 12 million killed, missing or captured.
McHrozni