Use of vital dyes for identification of lymphatic vessels before cannulation has heretofore not been approved by the Food and Drug Administration (FDA). The suitability of isosulfan blue, the 2,5 disulfonic acid isomer of Patent Blue, for this purpose was evaluated experimentally in the rat and clinically in 11 volunteers and 543 patients under an investigational new drug application. FDA approval for this drug has been obtained. Volunteers and patients received up to 15 mg of a 1% sterile, pyrogen-free solution per extremity (total dose of 0.4 mg/kg in the average patient). Excellent identification of lymphatic vessels was achieved in 100% of volunteers and in 97.4% of patients. In the other 2.6%, lymphatic vessels were not identified mainly due to congenital lymphatic vascular (Milroy) disease. Baseline blood chemistry in volunteers was not altered after administration of the dye. No adverse reactions were found in volunteers and minimal allergic reactions occurred in less than 1% of patients. Acute toxicity studies demonstrated an LD50 greater than 150 mg/kg in the rat. Isosulfan blue was excreted unchanged in the urine (7%) and feces. Comparable excretion was found in volunteers. Isosulfan blue is a safe and efficacious vital dye for lymphangiography.