The radiologic and pathologic effects of canine renal embolization with dextran microspheres of two size ranges (40-150 micron and 100-300 micron) were studied 30 minutes and 1, 2, 4, and 6 weeks after embolization. Dramatic occlusions occurred with very small quantities of these particles, which reached more distally and were suspended and embolized more easily than other solid embolic agents. Because of these qualities, however, they should be used with more caution. They can be injected through 2-F catheters, calibrated-leak balloons, and open-ended guide wires. Embolization with dextran microspheres was also used for epistaxis, meningioma, dural arteriovenous malformation, renal tumors, bone metastasis, multiple myeloma, or peripheral angioma in ten patients. The technique was safe and effective, but special attention should be directed to proper dilution and gentle embolization with careful monitoring to avoid excessive embolization or reflux.