I believe this may be an [East?] Asian thing; not just Japanese. I know that Koreans also put great emphasis on blood types[1]. One of the first questions you can expect to answer when meeting new people is "what is your blood type?"<p>As a westerner, I really find cultural elements like this fascinating.<p><i>(edit: I should have read the opening sentence before commenting; sorry about that)</i><p>[1]: <a href="http://www.koreanwikiproject.com/wiki/index.php?title=Blood_types" rel="nofollow">http://www.koreanwikiproject.com/wiki/index.php?title=Blood_...</a>