• The blue light entoptic phenomenon was used to measure retinal blood velocity in eight patients with chronic granulocytic leukemia, six patients with leukopenia, and matched control subjects. The retinal leukocyte velocity of the leukemic patients was 0.53 ± 0.26 (mean ± SD) mm/s, whereas that of the matched control subjects was 0.46 ± 0.14 mm/s. There was no significant difference between these two groups (power: 96% for a difference of 0.2 mm/s and 66% for 0.1 mm/s). There was also no significant difference between the leukocyte velocities of the leukopenic patients and control subjects (0.47 ± 0.19 mm/s and 0.55 ± 0.14 mm/s, respectively; 89% power for a difference of 0.2 mm/s, 59% for 0.1). There was a correlation between the leukocyte count and the number of leukocytes seen in the entoptoscope. The results suggest that retinal vascular autoregulation can compensate for changes in leukocyte numbers that might have been expected to alter retinal blood flow.