The carcinogenicity of Symphytum officinale L., Russian comfrey, used as a green vegetable or tonic, was studied in inbred ACI rats. Three groups of 19--28 rats each were fed comfrey leaves for 480--600 days; four additional groups of 15--24 rats were fed comfrey roots for varying lengths of time. A control group was given a normal diet. Hepatocellular adenomas were induced in all experimental groups that received the diets containing comfrey roots and leaves. Hemangioendothelial sarcoma of the liver was infrequently induced.