According to a Japanese institute that does research on blood types, there are certain personality traits that seem to match up with certain blood types.
TYPE O
You want to be a leader, and when you see something you want, you keep striving until you achieve your goal. You are a trend-setter, loyal, passionate, and self-confident. Your weaknesses include vanity and jealously and a tendency to be too competitive.
TYPE A
You like harmony, peace and organization. You work well with others, and are sensitive, patient and affectionate. Among your weaknesses are stubbornness and an inability to relax.
TYPE B
You're a rugged individualist, who's straightforward and likes to do things your own way. Creative and flexible, you adapt easily to any situation. But your insistence on being independent can sometimes go too far and become a weakness.
TYPE AB
Cool and controlled, you're generally well liked and always put people at ease. You're a natural entertainer who's tactful and fair. But you're standoffish, blunt, and have difficulty making decisions.