This work extends the clinical benefits of phased array transesophageal echocardiography with high detail and contrast resolution to include neonatal patients. We have built several prototype, 64-element, 7.5-MHz phased array transducers housed in 6.2-mm endoscope shafts for use with commercially available imaging systems. The acoustic design is standard, but the miniaturized packaging of the electrical connections was quite challenging. The endoscopes demonstrate very good structural resolution and excellent sensitivity for color flow imaging and continuous-wave Doppler. They have been used on patients as small as 1.9 kg, frequently as an anatomical guide during catheter-based interventions and during congenital heart surgery. Array test data and representative clinical studies are shown. (ECHOCARDIOGRAPHY, Volume 13, November 1996)