This monograph is chiefly concerned with the etiological implications of the racial and geographical differences in patients with cancer. The analyses are based on a study of racial distribution and peculiarities found during an investigation of 6,072 patients with cancer who were observed in 35,293 autopsies performed at the Los Angeles County Hospital from 1918 to 1947. They deal primarily with the 20 most lethal types of tumors, namely, those of the stomach, large intestine, lymph nodes, lung, breast, pancreas, uterus, intracranial organs, prostate, urinary bladder, gallbladder, ovary, skin, soft parts, mouth, esophagus, kidney, larynx, bone, and liver, and a few tumors of special interest (carcinoma of male breast and penis, chorioepithelioma, Kaposi's sarcoma, malignant melanoma, nasopharyngeal and cervical lymph node cancers, retinoblastoma, and salivary gland tumors). The three large racial groups (Caucasoids, Negroids, and Mongoloids), including white persons, Mexicans, Negroes, Japanese, Chinese, and Filipinos, composing the population of Los