Several studies from different regions of the world reflect a positive family history of esophageal cancer among patients with the same disease. The all age-standardized rates for the Turkoman of northern Gonbad district in the Caspian Littoral of Iran is as high as 160-180, and truncated rates reach 300-400 per 100,000, the highest incidence rates in the world. A total of 602 esophageal cancer patients (427 Turkoman and 175 non-Turkoman) from this high-risk region were interviewed for a family history of cancer, particularly cancer of the esophagus. Family members were identified as being related by blood or marriage, and the occurrence of esophageal cancer was compared between these two groups. The expected number of cases among blood affines to relatives by marriage has been determined by a formula which takes into consideration several social and ethnic factors; specifically, average family size, the proportion of first cousin marriages and the likelihood of aunts and uncles by marriage also being related by blood. One hundred ninety-nine (47%) of the Turkoman patients in the high-risk area reported a positive family history of esophageal cancer, ranging from 1 to 7 cases in a given family, and involving a total of 299 individuals suffering from the disease of which 244 (82%) were related by blood and only 55 (18%) by marriage. The overall ratio of blood to marriage relatives was 4.4. Forty-six Turkoman patients also indicated that a total of 62 relatives suffered from another cancer, of which 54 (87%) were related by marriage as opposed to only 8 (13%) who were blood affines. Among non-Turkoman (low-risk population) esophageal cancer patients, only 4 (2%) had a total of 5 relatives with the same disease, of whom 2 were by blood and the remaining 3 by marriage. The ratio of blood relatives to relatives by marriage for this group is 0.6, whereas the corresponding figure among the Turkoman population is 4.4. It is evident that there is a familial history of esophageal cancer among this high-risk, ethnic population in the Caspian Littoral.