To assess whether gastrointestinal factors can cause chest pain ("linked angina"), we studied the effect of oesophageal stimulation with acid on coronary blood flow in 20 syndrome X patients. Coronary blood flow velocity (CBFV) was measured with an intracoronary doppler catheter positioned in the proximal left anterior descending coronary artery. Acid stimulation produced typical anginal pain in 11 patients. Mean CBFV was significantly reduced from 7.3 (SD 4.0) to 4.4 (2.8) cm/s. Gastro-oesophageal reflux leading to reduced coronary blood flow may be a mechanism to explain linked angina.