Okay, this is a very simple mnemonic and I'm sure many people are already using it. Here it goes anyway. The blood groups associated with CA stomach and ulcer can get confusing so,
1. An ulcer is round, so it is more
common in people with group 'O'
2. CA contains an 'A' so carcinoma
stomach is more common in
people with group 'A'.
That's it :-p