OBJECTIVE: To determine the appropriateness of intention to treat decisions concerning coronary artery bypass grafting (CABG) and percutaneous transluminal coronary angioplasty (PTCA) for patients with coronary artery disease in The Netherlands. DESIGN: Prospective study of intention to treat decisions using a computerised expert system. SETTING: "Presentation" sessions in 10 tertiary referral heart centres in 1992. PATIENTS: 3207 consecutive patients: 1618 CABG and 1589 PTCA candidates. MAIN OUTCOME MEASURE: Percentage of invasive treatment decisions rated appropriate, uncertain, or inappropriate by the expert system. RESULTS: PTCA decisions were common for patients with one-vessel disease and CABG decisions for patients with three-vessel and left main disease. PTCA decisions outnumbered CABG decisions in acute myocardial infarction. Of CABG decisions, 84% were rated appropriate, 12% uncertain, and 4% inappropriate. The proportions for PTCA decisions were 39% appropriate, 31% uncertain, and 29% inappropriate. Type C lesion was the main determinant of inappropriateness of PTCA decisions. If type C lesions were downgraded to type A/B lesions the rate of inappropriate PTCA decisions dropped to 6%. CONCLUSIONS: Clinicians in tertiary referral centres in The Netherlands favoured CABG if vessel disease was extensive or involved the left main artery, and PTCA for patients with less extensive disease and with acute myocardial infarction. Few CABG decisions were inappropriate. The main determinant of inappropriateness of PTCA decisions was its intended use in patients with type C lesions.