Background. Early reports indicate that transmyocardial laser revascularization improves symptoms in patients with refractory angina. However, there is little experimental evidence of whether blood now through channels is the mechanism of action. Methods. Endocardial channels were made in the distribution of the left anterior descending coronary artery in canine hearts (n = 5) using a holmium:yttrium-aluminum garnet laser. Hearts were excised acutely while perfused in a retrograde fashion from a second dog so that the aortic valve always remained closed. The proximal left anterior descending coronary artery was ligated. To measure direct transmyocardial blood flow, colored microspheres were injected into the left ventricular chamber. Results. The number of spheres per gram of tissue in the channel region was significantly higher than in the control region (low load, 302.5 +/- 169.0 versus 41.8 +/- 59.4; high load, 208.4 +/- 138.3 versus 5.8 +/- 11.7; both, p < 0.05). However, the estimated regional blood flow through the channels was extremely low (<0.01 mL . g(-1) . min(-1)). In the chronic setting (n = 4) (2-week survival), no now was detected through the channels, and the endocardial entry points were closed. Conclusions. Transmyocardial blood flow does not appear to occur through channels made with the holmium: yttrium-aluminum garnet laser. It remains to be determined whether this is the case with other types of lasers.