PURPOSE: Increased leukocyte-endothelium interaction have been suggested as a phenomenon contributing to capillary occlusion and/or rupture of the blood-retina barrier during human retinal vascular diseases. This study was performed to evaluate if fluorescein-labeled autologous leukocytes (FLALs) can be used for examination of leukocyte transit in the human retina. METHODS: The preparation consisted of human dextran-separated leukocytes mixed with fluorescein. After reinjection in normal subjects and in one diabetic patient, a confocal scanning laser ophthalmoscope was used to visualize them in the retinal circulation. The changes between FLALs and control leukocytes in the expression of leukocytes adhesion molecules CD11b and CD62L were evaluated by flow cytometry. RESULTS: The circulating FLALs were clearly visible in retinal vessels. The mean (+/- SD) capillaries velocity was 1.43 (+/- 1.3) mm/s in the macula and 1.82 (+/- 1.4) mm/s in the peripapillary area. No leukostasis was detected in the normal subjects, while it was detected in te diabetic patient. Flow cytometry revealed an increase in CD11b and a decrease in CD62L expression of leukocytes after labeling, suggesting that compared to normal leukocytes FLALs are more susceptible to interact with vascular endothelium. CONCLUSIONS: The use of FLAL is presently the only technique applicable in humans for study of leukocyte transit in the retina. Their preparation is technically simple and unexpensive. Precise measurement of the velocity of leukocytes in small vessels can be obtained. Despite evidence of a certain degree of leukocyte activation after the labeling procedure, no leukostasis was detected in vivo in normal subjects. Potential applications for this technique may include the detection of leukostasis in the human retina during severe forms of diabetes and retinal phlebitis.