We developed a system that permits exceptionally accurate measurements of human choroidal thickness in vivo by means of ultrasound. With minicomputer techniques, the complementary functions of the time and spectral domains of reflected sound can be used to permit measurements accurate to less than 20 mu at 10 MHz center frequency. Even more accuracy can be obtained with higher center frequencies. Choroidal thickness has been found to exceed that previously reported, with typical values of 420 mu found at the posterior pole. Even greater values are seen in regions outside the macula.