Clopidogrel pre-treatment before planned percutaneous coronary intervention was proved to reduce periprocedural complications. However, the vast majority of patients in the current interventional cardiology practice do not undergo planned PCI, but rather "ad-hoc" PCI performed immediately after coronary angiography . Whether clopidogrel should be administered as pre-treatment to all patients undergoing elective CAG with the aim to ensure therapeutic levels at the time of possible ad-hoc PCI is not known.

Inclusion Criteria:

1. Age ≥ 18 years
2. Elective CAG for suspected or proven coronary artery disease (stable forms or fully stabilized acute coronary syndrome)
3. Signed written informed consent

Exclusion Criteria:

1. Thienopyridine treatment in previous two weeks
2. Contraindication for clopidogrel
3. CAG scheduled less than 6 hours after potential randomization
4. Clinically significant bleeding (i.e. with hemoglobin fall by > 50 g/l and/or requiring transfusions or surgery) in previous 3 months.