In patients with typical stress-induced anginal pain, normal coronary arteries, and unimpaired left ventricular performance at rest ("syndrome X"), a reduced coronary dilatory capacity, abnormal lactate metabolism during stress, and reduction of left ventricular functional reserve have been described. A group of 40 patients with syndrome X was followed for several years to determine their long-term prognosis. In 27 patients pulmonary artery pressure and in 19 patients left ventricular ejection fraction were reassessed during rest and exercise approximately 4 years after the initial examination. In patients with stress-induced ST-segment depression, these variables did not change during the observation period. In patients with constant or rate-dependent left bundle branch block, however, there was significant deterioration of left ventricular performance during rest (pulmonary artery mean pressure, 16 +/- 3 vs. 17 +/- 4 mm Hg, p = NS; left ventricular ejection fraction, 62 +/- 5% vs. 55 +/- 5%, p less than 0.05) and exercise (pulmonary artery, 30 +/- 6 vs. 39 +/- 10 mm Hg, p less than 0.005; left ventricular ejection fraction, 59 +/- 6% vs. 49 +/- 5%, p less than 0.01). These findings suggest that in syndrome X two subgroups with distinctly different prognoses may be defined: In patients with stress-induced ST-segment depression during exercise, left ventricular performance remains well preserved; however, in patients with either constant or rate-dependent left bundle branch block, there is significant deterioration of left ventricular function within several years.