Which are the most common blood types?

Worldwide, the commonest ABO blood group is O, and the commonest Rhesus group is positive. The second most numerous member of the ABO groups is A. Therefore, mathematically, the next most frequent blood groups must therefore be B and finally AB. (If you're interested in the logic behind this assertion, leave a comment to that effect and I'll answer it there.)

If you want nice round figures, I can offer you this approximation:

• O: 45%
• A: 40%
• B: 10%
• AB: 5%

But if each ABO group is divided into a rhesus +ve and -ve component, the pretty figures get quite messed up. It is always far more common in the general population to be rhesus +ve than -ve. In white people, around 15% are -ve, whereas in native black and asian people (as opposed to those living in, say, America, where some sexual mixing between the races has occurred), it is extremely rare to be rhesus negative (<0.3%).

The above figures can vary significantly depending on which country is measured. For a look at the figures for several 'common' countries, you can go to a Wikipedia page here.