Coronary vasoreactivity of patients with chest pain syndrome (CPS, 18 patients) was examined with intracoronary acetylcholine infusion test (ACh). For comparison, 10 patients with vasospastic angina (VSA) and 17 patients without chest pain (control group) were used. The luminal diameters of coronary arteries were measured before and after ACh, and the maximal value of constriction rate of each segment (MCR) was used as index of vasoreactivity in each patient. By the ACh test, an average MCR of 42 +/- 23% was observed in CPS, 84 +/- 17% in VSA, and 26 +/- 12% in the control group. In CPS, chest pain was induced by ACh in 7 patients (group I), but was not induced in the other 11 patients (group II). The average MCR of group I (66 +/- 18%) was significantly higher than group II (28 +/- 9%, p less than 0.01) and the control group (p less than 0.01), though lower than VSA (p less than 0.05). These findings suggest that increased coronary vasoreactivity may play an important role in the chest pain development in CPS.