Vascular malformations of the GI tract can be treated by thermal methods, including the YAG laser. The diagnosis of these lesions is not always easy, and it is difficult to evaluate the completeness of therapy of these lesions. We investigated the value of Doppler ultrasound in this respect. The TVD-1 (Key Med) transendoscopic vascular detector using the 2.5 mm diameter probe emitting at 7 MHz, was used through standard endoscopes. Outputs were monitored through a loudspeaker, and imaged on an oscilloscope, or recorded on a mingograph. Diagnostic usefulness was studied in 64 vascular lesions, 36 in the colon and 28 in the upper GI tract in ten patients. A Doppler signal was detected in 75% of the lesions, including all lesions larger than 5 mm in diameter. Forty-five lesions in 13 patients were studied before and immediately after laser therapy. Three out of five patients in whom Doppler-positive lesions persisted after laser treatment rebled, as compared with 2 out of 8 patients with Doppler-negativity. Disappearance of the signal, however, may be transient, and is probably due to laser-induced edema. These results suggest that endoscopic Doppler ultrasound may have a role in the diagnosis and treatment of vascular anomalies of the GI tract, and should stimulate further research.