OBJECTIVE To investigate the value of coronary pressure derived fractional flow reserve (FFR) measurements in supporting decisions about medical or surgical treatment in patients with angiographically equivocal left main coronary artery stenosis.

DESIGN A two centre prospective single cohort follow up study.

INTERVENTIONS FFR of the left main coronary artery was determined in 54 consecutive patients with angiographically equivocal left main coronary artery disease. If FFR was ⩾ 0.75, medical treatment was chosen; if FFR was < 0.75, surgical treatment was chosen.

MAIN OUTCOME MEASURES Freedom from death, myocardial infarction, or any coronary revascularisation procedure.

RESULTS In 24 patients (44%), FFR was ⩾ 0.75 and medical treatment was chosen (medical group). In the remaining 30 patients (56%), FFR was < 0.75 and bypass surgery was performed (surgical group). Mean (SD) follow up was 29 (15) months (range 12–65 months). Survival among patients at three years of follow up was 100% in the medical group and 97% in the surgical group. Event-free survival was 76% in the medical group and 83% in the surgical group.

CONCLUSIONS FFR supports decision making in equivocal left main coronary artery disease. If FFR is below 0.75, the decision for bypass surgery is supported. If FFR is above 0.75, a conservative approach is justified.