Clinical studies of TMR have noted a relatively high incidence of cardiac complications in the early postoperative period (1- 8,13,17- 19), with the majority of this morbidity occurring in the first 24 h after surgery. Several authors (5,8,13,19) have hypothesized that these adverse cardiac events may stem from myocardial edema after the procedure although this concept was previously unproven. In addition, early postoperative myocardial ischemia has been noted to occur relatively frequently post-TMR (18), and yet the possibility of laser treatment as a potential cause of this ischemia has not been described. Patients with preoperative unstable angina and congestive heart failure (1,8,19) have been noted to experience significantly greater post-TMR morbidity and mortality than stable patients. Based on the findings of this study, it seems logical that when laser induced injury is superimposed on significant preexisting left ventricular dysfunction, worsening heart failure with its associated morbidity results. In addition, these findings suggest that TMR with a holmium:YAG laser might be expected to produce greater early postoperative cardiac dysfunction than CO2 laser TMR due to the associated changes in both systolic and diastolic function versus diastolic dysfunction only with CO2. The published experience with holmium:YAG TMR to date is relatively small (5- 6) compared with CO2 laser (1- 4,7- 8,13), and, consequently, it is too early to tell whether the findings of this study will be reflected in the clinical setting. Finally, although the holmium:YAG laser appears to affect systolic as well as diastolic function in the early postoperative period, it is unknown whether these changes will have any adverse effects on the long-term functional outcome of the procedure. The experience with the CO2 laser suggests that short-term changes in diastolic function do not adversely affect the clinical outcome and benefits of the procedure. Randomized trials comparing the CO2 and holmium:YAG lasers have not been performed, and further research in this area is needed.